Hi, I'm new to the website so hopefully I'm doing this correctly.
I own a 1975 280z (original l28e engine) and for some reason it cannot turn over. I've changed out the fuel pump and fuel pump relay/EFI relay. Upon further investigation, it has come to my attention that none of the pistons are firing.

Thanks so much for the advice

Welcome to Nico!

The injection system on these cars is pretty antiquated, it's based on the old bosch jet tronic systems. You say "none of the pistons are firing". Have you checked for spark?

Step one would be to verify spark. if you have no spark it likely has nothing to do with the injection system as these earlier cars were not controlled by the computer. If you have spark next is to verify fuel.

Verify fuel pressure. Cycle the key from off to run position, the pump should come on for a couple of seconds to prime the lines, you should be able to hear it. If nothing happens the first suspect would eb the afm, sometimes the flapper gets stuck open, it must be closed and working for the fuel to prime. If you have spark and fuel pressure then you need to see if the injectors are firing

This is a bit more complicated to test, and will require noid lights, or a logic probe. Please let us know what you find or if you need additional help doing any of these tests.

I apologize for the tardy delay, I was incorrect in my belief of the pistons not firing. After testing the other options, we have spark, don't hear the fuel pump priming, and I believe I corrected the issue with the AFM.
We are testing the fuel pressure regulator tomorrow. Any other suggestions would be awesome!
Thanks so much for the helpful insight. Hope y'all are doing well with you're own vehicles!