[LMP]

I do not know what is the standard ratio with the '97 GP....however 2 points:
1 - if the transmission needed repairs, maybe the TCC (torque converter clutch) was inoperative before, and active now...which would account for a significant rpm drop at steady speed. 300 rpm drop at 60 mph on level road and no head wind is at least expected...I would have even expected a slightly greater difference....that I can see with my '93 Transport...but the GP has a better wind penetration so maybe the difference can be smaller.
2 - was your transmission actually repaired or did you have a transmission swap...? Several ratios are used with the T60E and my son's 2000GP GT has a slightly higher ratio than my TS...to enhance performance. 1650 at 60 is typical of my TS....but my son's GP (not supercharged) runs closer to 1900....but again, yours is a '97....there might be differences.