318, 383 or other? Which one

Based on what he's got already, I am willing to bet that it is already a high 12 to very low 13 sec car, at slowest. He may be close to running with an assumingly warm 454 in a Camaro. If not, a bit of head work or maybe a better flowing set of heads may do the trick.

He is probably making 350-375 hp at a minimum already.