I remember seeing someone with a Gbody that installed GTO seats front and back but it took some fabrication to get the rears to fit.
If my memory serves, that seat setup was on an '86 Monte SS, that was featured in Hemmings Muscle Machines years ago. He used the GTO seats in the front (they're a tight fit, but they do work, or so it says in the article), and he used the back seat from a '90s Cadillac Eldorado. The Eldo seat took some considerable modification, and he basically built a "hybrid" seat frame, using the original Monte back seat frame, and part of the Eldo rear seat frame combined, with new padding and a custom seat cover. I've looked at rear seats from early '90s FWD Cutlass convertibles, but I don't know if they're even close, and I have the back seat from a '93 Bonneville SSEi to try some time. I know that the Bonneville rear seat isn't like individual buckets, but it is much more contoured than a stock G body rear seat, and it is close to being the same width and height.