[Coupeguy2001]

As I remember it, when I rewired my 57 chevy truck,the harness came in 3 separate harnesses that plugged in together.
I got mine from a place in Arizona, and not sure what the name of it is, they were in Moreno Valley. ( I think)
every wire had a number on it, and it came with a diagram. Just match up the object with the number, and secure the wires.
It looked like to me that the underdash wires were separate, and the under hood wires were separate, and you plugged the two onto the main harness. Kind of like they made a main harness, and you plugged in the harness for your truck.
For instance if you had a V8 or 6, if you used an alternator or generator, or you had a 55-57 or 58-59.

When I put my overdrive and fan, and electric fuel pump, and stereo in, I made my own harnesses for those things and added a fuse holder for each system. I also went to auto zone and bought ten of those little 30 amp relays and wired them in for things like overdrive, fuel pump, stereo, 100 watt amp, cooling fan, etc.
There is plenty of room behind the seat if you use an under bed gas tank, kind of keep things away from under the hood, so it looks clean.

If you want to keep the in cab tank, try making an aluminum panel to mount your fuse block and relays to, and keep it high enough so you don't see it.
Buy a professional wire stripper and a professional crimper and color coded wire.
If you use relays, you can tie all the relay coils to the ignition switch and all the powers to a main battery cable that has a fuse holder block for each system.

You can make your own wire tags from those labelling machines, and it looks great. If you give them each a number, make your own list of accessories, and add the numbers to it.

You can even label each end of the wire.
For instance, 1 is battery to ammeter, and the ammeter end can be 2
3 can be ammeter to ignition switch, and 4 is the ign. end.
Or, 1 is battery to ammeter, 2 is ammeter to ign switch.
It really isn't that hard.
Say you want to add a cooling fan. the relay you want to use has a coil to actuate it, and a set of contacts to run the fan.
the ignition switch wire goes to the switch, and the switch wire goes to the coil positive fuse, and then out of the fuse ( .5 amp) to the coil positive contact. The coil negative goes to a ground point.
Then you have a lead from the battery to a fuse that runs the fan motor, say 10 amps, then to the fan. The fan negative goes to the chassis ground some where.

So what happens is that you are in traffic, and you want to turn on your fan. You flip the switch, turning on the relay coil, and the relay takes 12 volts on a red #12 wire to the fan, and a black #12 wire back to ground, and the fan runs.
If you want to incorporate the fan thermostat, it comes with the instructions on how to hook it up and where to place the sensor. To ensure long life for the thermostat, you use the thermostat to run another relay so it takes only a small amount of amperage to turn on the thermostat to run the fan.
simple and easy.
Just make a diagram, or list of the wire numbers, and have an area big enough for the relays. they are about 1 inch by 2 inches.
Most of them have push on contacts, so you get push on terminals and crimp them on the wires.
I like to use a 1/16" diameter rod where I want the wires to go, and use wire ties in a few places to keep the wires in check as I add wires, and make the bundle.
Then use a small screwdriver with the sides rounded off like the end of a spoon, and straighten the wires as you go, make=ing it neat and the wires straight.
if you start your bundle with a white, broun, red, and blue visible on the top of the bundle, you want to see those same wires on top through the entire run till they gt to where they are going, then terminate them with a service loop so you can change the part, relay, or wire if need be.
Wires need to be one size larger than necessary if run over 20 feet. Alternator to battery wire needs to be 6 or 8 gauge if over 36 inches.
relay cpoil wires can be 20 or 22 gauge, but the power wires need to be large enough for the items run.