I have solved the problem.
The Buick was never equipped with a fuel return line. The 2GC EFI conversion has a fitting for a return line. However, the EFI fuel pump (while a retrofit for the factory tank) does NOT have a return line, so I simply capped off the fitting on the 2GC unit. This caused a problem, putting the fuel pressure over 100 psi.
So, first I took my fuel pressure gauge OFF the Buick and installed it quickly on my Silverado, just to verify a working gauge. It held at 58 psi no problem. Ok, the gauge is good.
Then, just for sh1ts and giggles, I took the cap off the return line on the 2GC EFI unit, and rigged a hose from that into a bucket. Wife turns the key to run and BAM - 58 psi (but quickly falling) because there was no way to regulate an open hose into a bucket lol. If that line were in the tank, the pressure would stay constant.
There's no such thing as an EFI retrofit fuel pump WITH return line, for THAT VEHICLE. Have to either buy a whole new EFI tank with an aftermarket EFI pump, or, get creative with the existing EFI pump.
Stay tuned. I WILL get this working.