We have a freshly rebuilt 350 bellybutton in a '57 Desoto. Problem is above 4000 rpm's it bogs down like it's running out of gas. Rebuilt Q-jet and mild roller retrofit cam with new pump push rod. Fuel pressure at idle is 6 psi and drops to 2 psi when road tested at 4000 rpms. Volume at idle was checked, and it pumped 2 gallons in 3 minutes. Pump is new a/c Delco. Fuel sender has been removed and inspected, sits about 1/4 inch from tank bottom. Rubber hoses are new and in carb filter new. Full tank makes no difference. At this point should I try another pump, high volume? Ideas?