I think Mr fixit is closer to the mark here, not being familiar with this truck model myself I can say I'm am familiar with beam regulators and impco fuel systems. When old beam systems where used they had an inline fuel solenoid and filter combined these would eventually get filled with the heavy ends and clog up. Most impco systems I've come across don't have a fuel lock of solenoid but a vacuum lock off unit and filter that feeds the fuel regulator. Have you left the original fuel solenoid in the line when you changed to the impco units.

Regards Titus