First off, NICE GTO!!!!
I consulted ChatGTP and it said this...
"It's possible that the back seat from a 1971 Chevrolet Nova four door could fit in a 1971 Chevrolet Nova two door, but it's not guaranteed. There may be slight differences in the dimensions or mounting points between the two models that could affect the fit.
The best way to determine if the back seat from a four door model will fit in a two door model is to compare the dimensions and mounting points between the two seats. You may also want to consult a mechanic or a knowledgeable enthusiast who is familiar with these cars to get their opinion."