I've done lots of wiring.
My advice.
1. Do not try to build your own. You must use a prewired fuse panel, universal style harness.
2. Do not buy from an Ebay seller unless it's one of the trusted vendors. They also sell on Ebay.
3. The Speedway universal 12 or 22 will work fine.
I don't know your experience with wiring. The harnesses require crimping. A good tool helps a lot. Most harnesses support alternator and HEI. 12 circuits should be enough. Soldering of terminals is not necessary. You asked for "custom". Not necessary. "Universal" is what you want and seem to be looking at. Two different words with totally different prices.
The price difference of universal harnesses is sometimes justified depending on the "extras". I.E. Switches, extra connectors, lamp sockets, fuse panel style and so forth. So factor that in if you need those......but all that is available separately as you see the need later.
My Tips and futher suggestions:
1. The fuse panel is best mounted upper left of your left foot, inside cab.
2. Abandon the "terminal blocks" that were mounted on the inner fenders. Direct wire, using a soldered splice where necessary to crossover.. Use same splice for rear tail lights crossover/license light.
3. The wire bundles that go thru the firewall are routed down the left innner fender to front and down the left frame rail to rear.
4. Wire colors and quality are all the same for all the vendor's harnesses.
5. Crimping with heat shrink sleeving covering the "joint" is best.
6. Watch some vids on crimping provided by vendors: AAW, painless, speedway, EZ, etc
7. Use the braid type ground straps to get the motor, frame and body bonded to battery neg.
8. None of this is real hard to do.
9. It's your truck, you can do anything you want, any way you want, using any harness or technique you want.
Vendors= all the known companies we use that specialize in old, classic, vintage trucks.
