Losing fuel pressure on new setup

This sounds to me like vapor lock, or some problem in the suction side causing a restriction. I wold tee in a vacuum / pressure gauge (test vacuum / fuel pump gauge) on the inlet side. What do you have for a pickup screen, is it the typical sock? New? Old?

Also see what the pump will pull for vacuum on the inlet side with only the gauge on the inlet. Should be at least 12" Hg

Look in the service manual. There are procedures.

Just FYI I'm a HUGE believer in vapor return systems. Running a bypass regulator accomplishes this, with no regulator, you can use a Wix 33040 or 33041 filter, which has a 1/4" hose fitting with a return orifice, built in. You of course need to build a return tube back to the tank. Factory (440/ hemi) cars used 1/4" but I would run 3/8