Check to see if you have the 4 atmospheric breather tubes blocked off on the fuel tank (from the underside) I blocked mine off when I installed the tank on my 70 Charger and I lost fuel pressure because the fuel tank was sucking it back from the motor. Make sure you have proper vent lines. Drive around with a 1/4 tank of gas and some duct tape with holes in it on the gas cap area and see if the car runs right. Late at night so fuel doesnt go sloshing in and out of the neck tube.