making wiring harness from scratch

Matthon wrote: "What gauge wire is a typical a-body wiring harness?
Is it all the same gauge? (dash, under hood, etc) What gauge is the starter, battery, alternator wires? Just want to make sure if I am going to go up a gauge that I know where I am starting."

The various wires are different sizes, depending on the amount of current that they must carry. My suggestion would be to get a wire gauge at the hardware store, or use a calibrated wire stripper, to determine what gauge the various wires on the old harness were.

As far as the main power line is concerned (connects the battery, starter and alternator): of course it's larger than the rest. The bigger the better, because this wire has to carry the alternator's charging current. However, if you use a wire that's TOO big, it will be so stiff that you may have problems with something breaking at the alternator due to engine vibration.

"I think the bulkhead connectors are the same as the other connectors throughout the car, is this correct?"

The old bulkhead connectors will clean up just fine, and there is really no reason not to use them over again. Just clean and polish all the brass contact fingers until they are shiny, and do the same to its mating connector.

Again, it is not critical what kind of wire you use, or what kind of insulation it has, if you wrap the entire harness in fibreglass heat insulation tape, like the kind used to wrap headers. Just avoid wire with so-called "green" ("environmentally-friendly") insulation. (Not normally a problem because I've only seen that kind of wire used by the OEMs). That kind of insulation will break down after 15 or 20 years, no matter what you do to protect it. Mercedes and Volvo, for example, have had a big problem with this. Of course such problems arise long after the warranty has expired, and by that time the labor cost to replace the harness is often considered prohibitive by the original owner. I have bought two DOA cars this way really cheap and repaired the old harnesses myself, as I discussed in the post above. I'm driving one of these cars (1987 Mercedes 420SEL) and my ex got the other one (1980 Volvo 242) in the divorce and is still driving it.