I've done a ton of these interior swaps.
I've got a nice set of the grey leather bizzilion way power seats from a 87 Riv T-Type that I aligned in the car, measured, and then fabbed and welded tabs to the sides of the tracks for the new attachment needs. I didn't match the rear for that car, It looked close enough for me already.
Another Buick job is the full interior swap using 86 or 87 LeSabre T-Type plush cloth power seats in my 71 Rivi. (black and grey, look like a Regal Limited)
The rear is where it gets tricky. First of all, for a matching interior you choose your front seats by your rear width dimension. You can fudge it an inch or so wider than you need and stuff it in, but shorter doesn't look good, even with filler panels extending your interior trim. You'll need some steel wire that you can bend up to match the top locating hoops for the seat back. Just cut or tear out the foam from where you need the new hoops, weld them to the seat frame and throw a wet towel over it, or some water from a 5-gal bucket standing by. You're sure to burn some foam while mig'ing them in place.
For attaching the bottom of the back, it's best to just drill a couple new small holes in the floorpan, or just leave it loose. A modern rear seat base is thinner in height from our old stuff. You'll need to buy some foam to take up the difference undeneath at the sewing store/ craft store. Get high density foam. An electric carving knife will trim it to suit easily enough. Face that lower extension with matching fabric to the seat or carpet. Attach with spray glue from the automotive paint store. Make hoop extensions for the new base that to connect with your factory hooks. Attach the new foam under the base with spray glue after cutting holes for the new attachment hoops to pass through.
SHAZZAM!!! New interior in an afternoon!
