I ran into this issue a few years ago. I have 2 different A/G bodies. One is a 1984 Monte SS, and the other is a 1980 Malibu. When I bought the Malibu, in the process of getting it running, I replace the fuel pump. The Malibu has a 2 line fuel pump, inlet from the tank, and outlet to the carb. I had the carb for the car rebuilt, and after the repairs, I had an issue with fuel literally flooding out of the carb. I thought it was a problem with the carb, but the rebuilder told me that he had seen this problem before. He suggested that first, I try gravity feeding the carb, and seeing if the car ran OK, which it did.. He told me that there are 2 different fuel pumps for a SBC, and they operate at different fuel pressures, because they have different diapragms in them. As he explained, there's a fuel pump with 3 lines on it, and a deeper bowl. One of the lines on that pump is a return line to the tank, because that fuel pump operates at a higher fuel pressure. My Monte SS has the 3 line pump on it, and there are 2 lines that run along the right side of the frame-a pressure line and return line. The Monte still has a separate line along the left side of the frame, that goes to the charcoal vapour canister. The carb rebuilder told me that a 2 line pump runs at 6-7PSI of fuel pressure, while the 3 line pump runs at about 10-11PSI, which is too much fuel pressure to the carb, unless there's a return line, which bleeds off the excess fuel pressure. He said that he had seen (in rare cases), where a 2 line pump had a 3 line diaphrapm in it, making it run at higher fuel pressure, which will cause the carburetor to flood. He told me to get another fuel pump, from a different supplier, and try it on the Malibu. I did. Long story short, end of problem. Never seen that ever before, but its the truth.
