I had a '77 Chev pu with a 250 and a monojet that would idle perfect but not drive, it was the filter in the carb's fuel inlet plugged up. Also check that you don't have a bent or clogged line from the tank to the pump, and that the tank outlet is clear. Take an air hose and blow back from the fuel pump inlet fitting with the gas cap off to check for free flow, sometimes sediment will accumulate in the tank.
Tim
