scrappie

Can anyone tell me for sure if the rear quarter glass for a 71 cutlass conv. is unique? I am trying to finish up a resto and can't seem to get the glass to seal tight against the top. It was like this when I bought it but I just thought it was out of alignment. I have spent hours trying to get it to fit right but have had no luck. All the other glass seems to fit pretty well. I am thinking maybe the previous owner replaced it with a piece out of a cutlass supreme body where the glass looks similar or is exact I don't know. I am just trying to rule the glass out before I start looking elsewhere. Thanks to all.