I have put a 350 and a vette 700R4 in front of a stock V6 rear. I believe it is an 86 3.73 rear with open carrier. My question is can I expect it to last in comparisson with the stock V8 rear? (beefier parts??) and what does it take to go from open to Posi? Cost and parts in particular?

No difference between 6-cyl & V8 rear ends.

You need a posi carrier with the right spline count; which would be 26 splines, if the rear is really from the 86 car. And some fluid, and a bottle of posi additive.