The 88 door will not fit. It is a totally different door...not only the seatbelt mounting but I believe the lengths are different. Same thing happened to me when I unknowingly bought an 88 regulator for my 90. The 88 is 2 inches longer than my stock 90. The seller gave me my money back though...maybe the guy who sold you yours will do the same because I think in order to get it to fit (if there is any way in hell), it would more work than it's worth.
If that doesn't work,...just get like 2 fat rolls of ducktape. Duck tape around your car so the door is supported and just climb in and out of your window - Dukes of Hazard style! YEEEEHAAWWW!