My current situation I have a 66 Belvedere it's a restomod I have a 6.1 hemi w/ the 727 torque flight behind it. Because it's fuel injected I purchase a Rock Valley fuel tank with the fuel pump built into the tank. This tank also has walls built into it so that fuel is always around the pump and the vehicle is never is allowed to get below a 1/4 of tank.
My problem this pass weekend while driving the car and it seem like it was giving out fuel so i pull to the side of the highway to check everything out and the car shut off. I tried to restart it and it would prime to 70 psi then run for 30 secs then shut off. I added more fuel knowing it didn't need it but at the time trying any and everything to get it back running. I straight wired the pump to see if it was a power issue but the same problem. I call a roll back and took it home. Replace the fuel pump and in the beginning I couldn't keep it prime while running now I can't stop the pump from pumping to much fuel.

Setup
Aeromotive 13109 - Aeromotive A1000-6 Injected Bypass Regulators
Pump replacement LS style internal electric fuel pump for E.F.I. Unit/60-65 PSI order from rock valley to much sure i got the right pump.
Fuel leaves the pump goes threw a fuel filter to my fuel regulator on this regulator i have a pressure gauge also to measure how much fuel goes to the rail.
Fuel goes from the fuel regulator to the fuel rails. On the fuel rail i have a pressure gauge and once the pump prime the pressure gauge jumps to 100 psi then blows the O-ring from the rail.

I've tried holding down the rail and fuel started leaking from the regulator. Once i release the rail blows fuel from the injector directly under it and stops leaking from the regulator. Ensure all fitting are tight and every time this happen. It's so quick to happen the regulator doesn't read the pressure. I adjusted the regulator so no fuel should bypass but the same thing still happen. I open the regulator so it has no resistant and the same thing happen. The regulator has a built in return line so this is why everything is so confusing because all access fuel should return to the tank.

maybe the return system is clogged or otherwise blocked..?

Sure sounds like the regulator is stuck closed OR
the return line is plugged