A new loom would be a good, if expensive way to begin, but I'd find it rather gauling to have to start cutting into a brand new loom and modifying it as the original for whatever accessories have been fitted / removed over the years, or finding that bullet connectors on the loom had been substituted with spades or whatever on the equipment fitted to the car,
Also, as you dont want to pull the body off the chassis; bit of a bludger to weave into place, as factory fitted, loom goes on before body, and sits twixt chassis and body for largest portion of its run.
Not impossible to fit a new loom with the body on, but makes life hard for yourself.
Personally, I would make up my own loom; and I would start by getting a vehicle Wiring Products Catalogue...
Check my webby; thier addy is in my library & links section under Land-Rover 'Commercial' sites.
I would not be inclined to follow factory wiring runs or diagrams; I would scratch build my own loom, and my starting point would be at the battery.
I would get a weather proof electrical terminal box, to mount to the chassis under the battery tray, and I would put mount three bonding points into it. One for earth, one for battery live, one for ignition live.
I'd take a pair of cheap jump leads, and attach battery clamps to the ends, and attach them to a flat batttery, then neatly run them to the terminal box, and cut to length, then I would fit clamp on ring connectors to attach them to the earth and battery live pillars in my box.
The remaining jump cable, I would add clamped ring terminals to, and run one black wire from the terminal box, round the engine bay and up the right hand side of the engine, and attach to the alternator pivot bolt to ensure good earth return between the alternator and everything else.
I'd pribably use abother reterminated jump lead following similar path, round the right hand chassis leg, but take that all the way along to a starter motor mounting bolt.
And I'd then do like wise on the plussative side, running a red jump lead cable from the starter solenoid and the alternator output, back to mt junction box.
Rather a lot of extra copper in there, and run in parallel; factory do it with one lead of half the guage from alternator to starter, and starter to battery.
All well and good, but I like belt and braces engineering, and having just ONE junction makes life a lot simpler, and having GOOD known earth return paths saves a heck of a lot of head scratching later.
So, we now have the heavy cable in place; the alternator feeding the junction box, that feeding the battery and the starter, with isolated earth returns so that you dont have to worry TOO much about faulty earths or high earth resistances.
But, I'd use the earth bonding terminal to bolt the junction box to the chassis, so acting as an earth bonding point.... but belt and braces, would run a brad off the back of it, to the water pump; and another to the bulkhead & hence body)
out of VWP catalogue, a 60A relay between the battery live terminal and the ignition live terminal, inside the junction box, and you should have good starting point for the rest of the loom, with no dangly threads piggy backed into the battery clamps.
Next thing to decide upon is the fuse box; for convenience, tempting to use a blade type fuse & relay board from a modern car.... the one from a latish Range Rover would imedietly endear itself to me; the fuse board having LOADS of spaces for fuses in it (though no relay slots), and it would conveniently fit in a sandwhich sized tuppaware.....
So I would procure one, and bung it in a snap lid box, and aim to glue that to the bulkhead.
From that, I would work out what circuits I needed or wanted; though being belt & braces, I'd want to relay all the high powewr circuits like headlamps, and if fitted rear heated windscreen.
Not too much electricker in a series as standard, so I'd probably PLAN on wiring circuits for all teh stuff I DIDN'T have, but MIGHT consider fitting at a later point, like reverse lights, work lights, front & rear fogs, front spots, roof lights, Radio, cigarette lighter socket, anything like that.
I would make up a sub-loom for the main lights; headlamps, indicators, side lights and brake lights, and conveniently, there is a proprietry multi-core cable with seven leads in it for that exact purpose, sold for wiring trailer boards.
From the relays in the tuppaware, three strands of 7-core out of it from a common terminal rail; one to each front corner and one down the chassis and up through the floor for the rear offside corner, a spur from that back out and along the cross meber, to the rear nearside corner, a spur off that, back to the middle for the towing socket.
I would use the earth cabe in it as an earth cable, for everything the rest of the cable was attached to to be sure of good earth returns for them, and that would be commoned to the earth bonding point in the junction box.
(Geuss you figured I DONT like relying on the chassis or body to give a good return path!)
Connectors, EVERTHING soldered, NOT crimped.
Use VWP catalogue but avoid crimp-ons like the plague, where you HAVE to use them remove the plastic insulation from the connector, slide it up the wires insulation, and solder the wire into the connector, then slide the insulation back on, and crimp if you must.
Use heat-shrink sleeve, and a good length of itto help support jounts onto connectors rather than tape or rubber boots, and test connectors for continuity to avoid dry joints before insulating.
Make sure you use the correct, or better guage of wire than circuit needs, and DEFINITELY use colour coded wire, preferably as the factory wiring diagrams, to make fault finding possible later.
Multi-block connectors are dead useful, especially where theres restricted access; make up spurs to go where you need them, and bring them to a common and conveniently placed multi-block on the main harness.
EG; when you do the headlamps, side lights and indicators in the wings; one trailer board cable through the inner wing with a multi-block on the end, inside the inner wing, so that if/when needs be, you can unplug that whole assembly from the car and remove the wing as an entireity, should you want easy access for an engine removal, or need to swap a wing from an off roading incident or anything.
Likewise a multi-block on the spur you send to the back lights, and possibly one between the corners.
Insulation tape 'wrap' when making the cables into harness is good, but plastic cable guide conduit is cheap, and adds protection.
Use it particularly around the engine bay to keep cables away from anything moving or hot, rather than stringing between clips.
Tip: aluminium corrodes around steel, and cable clips pop out eventually. Use split cable conduit, and to attach to body/chassis, drill two small holes opposite the split, through both conduit and panel to be attached to. Use a prit pad to stick conduit in place and act as a cusion for it, then thread a cable tie throught the holes to secure in place.
For heavier cable, or as clips, use plastic number plate screws, and drll through wide cable ties, to make an attachement point.
When you come to look at the headlamps, chances are if you have steel bowls the assembly inside will be a corroded mess; and the lucas side and indicators if you remove them will probably reveal a rotten triangular hole, rather than the neat centre hole and three attachement holes.
Not even worth thinking about re-using old indicators, side and stop/tail lamps; they are only about Â£6 each get a new set.
Headlamps, with bowls and bezils and everything rather more expensive, and add a Halogen conversion kit into the bargain, best part of a ton; BUT for the hassle it will save, well worth while.
Make the attachement points as good as you can though.
For the side, tail and indicators, my preference is to neither try and fudge the new units into the existing holes or try and make them as new with plates or anything. They will only continue to rot, or rot like they have.
Make up a pair of sandwhich plates from suitable plastic. For the outside, immedietly under the lamp and in cosmatic view, a 2mm thick piece of perspex, in black cut in a concentric circle about 1/2" larger than the light base, wont look out of place, and will spread the area that the lamp attaches to. On the back, a larger and less elegant and slightly larger bit of perspex can be used.
Push the light unit through the existing hole, with front plate covering the messy aluminium; place back plate on from behind, glue roughly into place with silicone sealant, aliggn then use supplied screws to clamp the two plastic plates together clamping the assembly to the wing panel; hopefully avoiding any more aluminium to steel contact, and corrosion.
Light unit will be denied electrical contact with the metal of the body though, so make sure you use the earth return in the sub-harness as advised.
Cant really think of much else. Done to a standard without cutting corners, job is frighteningly expensive and makes getting a proprietry loom look better value; BUT its the 'niggly' stuff like light units falling to bits when you disturb them that start cranking up the costs; and its very tempting to cut corners using say three or four reels of coloured cable, rather than a different code for all........
Just ask yourself WHY you are doing the job in the first place though; becouse so often it is the frustration of trying to trace a wire that changes colour, or having a myriad of wires all red or all black, going every which way, and not being able to figure out what goes to what, that made you want to tidy it all up!
That and, bad earths, and dodgy (normally crimped) connections.
So; right wire; right colour; right conections PROPERLY jointed, and EARTH every frigging thing, every frigging where!
Best of luck!