with the car idling, take a pair of pliers and pinch the return line coming off the FMU(restricting all fuel from returning to the tank and forcing the fuel pump to reach its highest possible PSI. The Walbro 255LPH-HP maxes out at 120psi (I looked it up for you) so if the pump itself is working fine, it should peg the needle on the pressure tester (assuming it goes to 100psi). If not, you either have a serious blockage between the fuel rail and the tank....or your Walbro is crapping out. I've had 2 Walbro Gss342's go bad on me in my past with the same issue you're having so its possible. I believe its the check valve inside the pump assembly which causes this, not the pump itself. My guess is it will max out at 90psi.

Do that and report back.

P.S. your static fuel pressure on your setup should be 60psi (car idling with the vacuum hose going to the FPR disconnected and plugged). at idle, with the vacuum hose connected to the FPR, it should be between 49-55psi (the FMU inline can sometimes cause some restrictions so it varies between these points depending on the setup). At 6-7 PSI you should be closer to 90-95psi.