Since I have done a IRS to leaf spring conversion about 50 years ago, if memory serves the rear frame rails are offset to one side. I know I used the rear crossmember from the donor car to attach the shackles to because of the frame rail offset. I had to notch the crossmember to fit over the Tempest frame rails so the leaf springs ran straight back from the front mounting points.
Donor car was a 1962 corvette, and the original fuel tank wouldn't fit under the car after the conversion, I used one from a 62 chevy, and had to move it into the trunk area.
BTW did you mean a 1980 F body instead of a 1990?
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Brad Yost
1973 T/A (SOLD)
2005 GTO
1984 Grand Prix
100% Pontiacs in my driveway!!! What's in your driveway?
If you don't take some of the RACETRACK home with you, Ya got cheated 
  
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