Fuel pressure cause a no start?

What is your question?

:key on=48psi then slowly drops to 43psi (takes 10 seconds). Engine on=54psi/idle, rev it up & drops to 48psi, let off & back to 54. Will crank, won't turn over without starting fluid. 130k miles, daily work vehicle. Changed: distributor cap, rotor, plugs, wires, tbs, speed sensor, battery, o2 sensors, fuel filter...checked all relays/fuses..scan showed p0308 & bad o2 sensor was all. I have a new pump, I just want to make sure before I make a $300 mistake.