[tdangle]

I agree it is a fuel issue. Only focus on that. you didn't mention what carb is on the 305, but I'm guessing the stock 2 bbl or 4 bbl. Most of these have fuel filters in the inlet., Double check or remove it for testing. I agree upgrade the tank pickup and lines to 3/8 as supplied with our V8 trucks.

Another test is does it fall flat on it's face on a full throttle run? that would confirm a fuel issue. the fact that if you leave it set and then it's OK for a bit tells me the carb bowl fuel level is on the low side. By the way has the carb been gone thru or has it been sitting for a while. Might be time to refresh.