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How far out is it ? It could be a bent floor pan from years of abuse...Try putting a floor jack under the inner rear mounting studs and jack the car up...
Do you have repo floor pans? I am having the same issue, but on mine the seat will not sit on all 4 corners. It only sits on 3. In other words it rocks back and fourth like one of the holes is too high. Not sure if the tracks are messed up or if the repo pans aren't correct. This is the full amd floor pan. The seats came out of a cuda, so I have never had a chance to see if they sat correctly on the floor pan of my challenger. I converted my challenger from a 74 to a 70 and chaged the seats.
most likely the seat back frames twisted, which give the inward lean look, i have seen a lot of this, only way to fix it , is to remove and disassemble the back and straighten the frame.
Had that problem on my passenger seat, Po recovered the seats and installed the inside seat track backwards. they are 1 5/8" high in the back and 1 1/4 hi in the front. check yours.