72 360 first start question

What type fuel pump, mechanical or electric? I just went through this with my brother-in-law's car. It was doing exactly what you described. After basically rebuilding the entire engine, we tracked it down to a bad fuel pressure regulator. He is running a Holley electric pump and the reg. **** the bed and was giving it too much fuel and it would backfire the carb and cause carb fires. Just a thought if you're running an electric pump. Good luck..