I also have FRPP 3.55's in my 91 auto, with a modified Eaton Positraction diff.
I'm not sure exactly what RPM I cross the line at (by that point I'm usually watching the water temp and A/F ratio), but I do know this - it's close enough to the red line that with 3.73's I'd probably run out of 3rd (Drive), and that just wouldn't work. So I'm glad I didn't get the 3.73's. My tires are BFG DR 255/50-16, I think around 26" tall.