Well now you say he has a 255 in-tank and a T-Rex with big rails, FPReg, and 42's, Now to me that should be fine, problem must be with the Hitachi wire sensor on the Pro-M, it could be bad or calibrated wrong, I've known Bob Atwood owner of Pro-M since the early 80's ( he died recently) and they are a top notch company BUT mistakes happen, you could have accidentally gotten a hot wire for a 24lb set-up and so forth. Call them, tell them your problems, they will tell you the proper proceedures to test it.
There are 2 grounds for the ECM, 1 is under the kick panel as stated above, and that smaller wire on the negative battery cable is the other, i know this cuz when i did my battery in the back, i forgot to ground that and the car ran like crap, if at all.
quick put some 19 pound injectors in it and a stock mass air, set fuel to 38lbs, see if it will run right, don't boost it, just drive it normal and see.
Also, in my opinion, I think you are running way too much fuel pressure, with those 42's you should not need that much pressure, but, hey!, your there and I'm here. Good luck!
