Fuel pump assembly replacement

I'm working on the car listed above Northstar 4.6. Fuel pressure test shows only 12 psi at the rail. Dropped fuel tank and was going to replace pump.
However, on this particular car (first design fuel pump assembly, tagged as ept), there is a fuel pressure relief valve integrated in the assembly. I can blow air back through the valve by mouth, and am not confident at all that it holds at least 45 psi.
I've checked all the parts houses I know of and the Chevrolet dealership; no one sells that valve or the 'first design' assembly. I can get the 'second design' assembly, which looks like it could be shoe-horned in, but no idea if it would match specs (level sender, actual output pressure).
Is this something you've seen before? Any ideas how to address this? Any knowledge of compatibility of the second design assembly?
Thank you for your time.