89406Iroc

I am running a 89' Camaro w/ 408,AFR heads, TPIS Miniram, 236/242 cam. etc. The problem I am having is that I will set Fuel pressure with vacuum disconnected. Set pressure to 45psi, when I do a pull driving Fuel pressure drops to 42psi. Check pressure at idle still at 45psi. Turn pressure up to 54psi at idle, do a run and drops to 50psi @ WOT with vacuum still disconnected. I figured that if I was loosing volume that I would loose more pressure at the higher FP.
What I have Tried, that has resulted in no change:
Disconnect return to check restriction. ( ran ext line for return)
Replaced Fuel pump from 255lph, to another walbro 255lph.
Changed Filter, installed T-rex inline , Checked Pressure before the rails.

Any Ideas? I am looking into an Aeromotive kit then doing sumped tank. But I figured that 255lph would handle it.
Thanks, Chad