Before just replacing the pump, have you isolated the problem to the pump? Try running off of a separate fuel supply to eliminate any hose/tank/filter/blocked tank vent problem. If you rebuilt the engine, did you get the fuel pump eccentric properly reinstalled on cam?
Most of the guys running open engines end up using a standard fuel pump. My old Olds ran for 15+ years on a stock AC Delco automotive pump with no problems. Might still have it down in the garage........