I fit late model xj seats, and late model xj seat belts in my '90 4door xj. The belt mounting locations are differant, when bolted up, the point where they latch together is lower than stock, which can make putting them on can be slightly annoying. I'd say the bigger you are the bigger of a problem this would be.
I prefered the newer style belts, but the older style latches are built better IMHO.