As far as the gage of metal..mike what you have and go with that..there are some conversion charts that will tell you what thickness a certain gage is..
the ole bird in the hand deal if you have the 350 sbc i would go ahead and use that..
Speedway motors is about the best source for parts for those in the suspension dept..Get the new parts as most of the old swap meet stuff I see is about junk..
Wiring..I think painless or EZ wire can provide a diagram with one of their kits..since the electrics will be new anyway might as well go that way..
I have tried most all of it and now do what is known to work..