I'm swapping out my 305 for either a 350 or 400 sb chevy motor...If I mildly built either one of these motors, do you think a stock 700r4 out of an 86 firebird would tolerate the torque?

 

On the upside, the '86 has the 30 spline input, and doesn't have the auxillary valvebody. On the downside, it doesn't have all the improvements that GM implemented for the '87-up 700R-4. How long it'll live is up to it's internal condition, and your driving style. I've run an '84 700R-4 behind a stock 350 for 20,000 miles with no issues. Most important is getting the throttle valve cable adjusted correctly. this is cheaper and easier with an '80-up Quadrajet, unless you're going EFI. It saves having to buy TCI's geometry corrector bracket and that other part, and there are different part numbers for Holley and for carter/Edelbrock. Secondly, while new fluid and filter is good, some shift kits are actually worse than doing nothing. The corvette servo seems to not hurt anything. some say it helps, but it's hard to prove either way. These earlier 700R-4s tend to break the 1-2 accumulator spring, so it'll be real weird going into second, but it's not a big deal, so long as you replace the spring right away.

 

After re-reading your post, I have a few more thoughts. Starting with a trans out of a firebird is probably not as good as a trans from grandma's caprice. Grandma might neglect maintenance, but she never puts her foot to the floor. Also, you have a COMP Thumpr cam in a 305? Which one, the smallest? How does that behave? At what RPM does it "turn on"? Would you choose the same cam again?