My conclusion-

I believe the non power V8 cured pitman arm has a much smaller spline hole to mount to the gear box.
I also think there's a slight difference in length

Here's my measurements-
Original Cast 9789158 (A GMT1)
Spline tapered hole = 1.178- 1.220 inches
Center spline to stud = approx 5.75 inches

Rare parts #20798 (V8 with power steering)
Spline tapered hole= 1.175- 1.220 inches
Center spline to stud= approx 5.50 inches

Rare parts #20761 (V8 without power steering)
Spline hole= 1.028 inches (according to manufacturer)
Center spline to stud= 5.663 inches (according to manufacturer)

The non power arm (#20761) appears to be closer to the 9789158 length, but the power arm (#20798) seems to be correct spline hole diameter...
I gave up and just installed the [censored] thing (#20798) with hopes that if someday this bird is drivable, I'll mess with it then if it's even an issue.....

One last thought- Why is number 9789158 not a correct part number? Is it because it's a cast number and not a official part number??