Re: purchased some leather seats for the cutlass
It was just last year and I'm already unsure what I paid for the reupholstery of the front seats... Senior moment in progress. Let's call it between $600 and $800, well over the cost of the seats themselves ($450, that I do remember).
There's info somewhere on the interwebs about installation of the GTO back seat in different cars. In most cases, it involved shortening the upper part (at the bottom) because they're too tall and fabricating fillers for the sides. Looks do-able but will be a chore.
As mentioned, I used an Eldorado back seat and the fabrication was ridiculous. The yard wanted pretty much the same money for the back GTO seats as they got for the fronts so I passed on them. The Cadillac back seat project was launched with a $40 investment which helped get that off to a quick start.