Either you have leak on the suction side, or a short push rod if this is a BB. If sb maybe you have the fuel pump installed wrong?

Just Saturday I installed a gas tank with new lines on a sb challenger. I only had to crank it for less than minute to get gas to the carb and for it to start after I poured 5 gallons in the tank.

Also I hope you don't have a filter in front of your fuel pump, because it can't suck through a filter that is designed to go after the pump. Only the sock in the fuel tank should be between your pump and fuel tank.