Let me see if I understand this...
You have the power-glide "auto that acts as a manual" transmission, correct? And you are going to put an auto U4P0 ECU in it?
I think it will mess you up. Everything else being equal, the auto ECU is going to expect your "manual auto" transmission to shift. So, it will do all the things it does when the auto would normally shift at 5200 RPMs. Same thing it would do if there was a T5 bolted in there, I believe.
Now, I do not know precisely what the computer tells the car to do when it is time to shift, but I would assume if you changed the shift points out to like 8,000 RPMs (just someplace where it never enters its shift mode while you don't want it to shift), then you would be good to go. I am thinking that, for example, with the auto ECU, you have the car in 1 and you are tooling along, you can always shift it manually into 2, even before you hit the WOT shift point, right? But, what happens if you are at WOT in 1 and hit 5200 RPMs without shifting? I don't know, to be honest. But, I seem to recall that if you have a T5 and you hit that point where it wants to shift you get all sort of issues.
Just throwin' that out there...