I have done a heap of hidden wiring on commodore engine bays and their looms are x3 the size of these older cars.
Take your time and plan ahead you need to get from point a to point b without going in the engine bay make shure you don't run wiring where it might rub like over a sharp metal edge or get wet like a spot where water drains or pools.
Insulate it electrical tape works well and use grommets or slip a peace of rubber over any metal edges where it runs over tie it out of the way with cable ties.
Label the wiring before you cut it eg 1a, 2a, 3a, 4a, 5a and so on , one loom might be for head lights and the other loom might be for the engine so label the engine loom 1b, 2b, 3b, 4b, 5b and so on so looms don't get mixed up.
when you cut wiring don't do it all in one go stagger your cuts so you don't end up with a big bulge of solder in one spot cut each wire at different lengths if that makes sense.
Solder and heat shrink every wire make sure the connection is clean of oil and dirt before you solder it, ensure your earths are a good solid connection to the chassis clean of paint and dirt, crimp, and solder on a ring terminal.
here a few pics
how i got it
this is what i found poor conections exposed wiring it would have taked alot more work to repair it.
this is what i did with it
got a new loom from a berlina
started to label this is an earth.
did as much as i could with it out of the car then put it in to get all the lengths spot on.
this is where i ran it in the guard across the top of the wheel arch
this was the engine bay earth
did the same with the engine bay loom
inside the guards securing wiring
also just be careful as wires can become brittle when exposed to heat and oil for a period of time i had to repair a few like this.
hope all this helped
