Can a stock fuel pump produce too much pressure?
My neighbor has a 57 Chevy, been sitting for a few years. Had a problem after he got it running of the 600 Holley carb doing the same thing you described. After he worked on it a few days, I put a fuel pressure gauge in the line to the carb. It had 12 psi of fuel pressure with the stock fuel pump. So it is possible that you have a fuel pump over pressuring the carb