A while back a a friend of mine put a BB in a 68 Camaro. It had about the same problem (he was also running a 2 1/2 in exhaust which is plenty). We went through a bunch of trouble shooting and finally one night took it out to a deserted strech of road and unhooked the exhaust from the headers. The car ran great. Turned out he had put on a cheap set of turbo mufflers that just wouldn't flow enough to let the engine run.