first off . check the modulator valve vacuume hose. make sure the detent cable is attached to the trans internally and free of binding. next spray a can of carb cleaner around the intake, base of carb and around the entire carb to see if there are any vac leeks.

and the used HEI is it vac advance type?
if so make sure that you disconect and plug off the vac source before adjusting the timeing.

how is the fuel pump? and fuel hoses?
once you have crossed out any problems with the engine, and still have the problem you may need to have the trans checked.

I have had a bad torque converter that had same problems that you have but I new there was nothing wrong with the engine because everything was new and had been tested and broke in at a friends Dyno. the only thing that I didnt replace was the trans and torque converter.


look over everthing again even if you had just replaced a part or adjusted something. check and varify everthing is where it should be and in working order. dot the I's and cross every T. you will find the problem. good luck I hope you get it going .