making wiring harness from scratch

Checking in on this thread.
A few questions as well:
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.

I think the bulkhead connectors are the same as the other connectors throughout the car, is this correct?
Also, I have seen the connectors on e-Bay.

I am thinking it might be best to replace the colored wires with the same color, but I have had zero success finding the correct colors wire. Any one have any suggestions?

Sorry for all the questions, but my wiring has been butchered and I am in the process of correcting it all.