Im having the same problem as you, and I also have a Walbro 255 intank. Ive changed the FPR and the fuel filter to no avail. Im seeing the same idle pressure as you also.

If I cycle the key 2-3 times before I crank it, it will start just fine.

Btw, your setup sounds awesome! Great #'s. Ive just got a lil 5.3 w/ a Whipple

 

That sounds like the same problem. I have the stock pump in there now, I need to install a new pump. Did you install the 255 intank in efforts of trying to fix the issue? Or are you looking at replacing your old 255? Sounds like you need a new pump aswell.