I picked up a set from a late model Gran Prix and I think they are going to bolt in. I actually got mine from a 4 dr. Only thing is that the latch from it bolts to the seat. I believe with a little mix and match it will work out though.
FYI.....Couldn't find seat belts for my 1976 El Camino w/bucket seats so I purchased a set for a 73/91 Suburban from LMC part 338-2246-BL @ $89.95 ea. They bolted right in to the same attachment points and have only one retractor. They work great and have a cleaner/smaller look than the originals.