Can i swap a 87 gt speedo in a 91 gt?

If you are talking about the entire gauge cluster with tach & etc, then Mike is right, 90-93 are a direct exchange. If you are talking about the actual speedo section, then you can possibly make the older one work in the newer cluster, but it would involve some cutting, filing and drilling to make it fit. I put a 92 speedo in my 87 cluster, so it shouldn't be too hard to swap the other way. But most of the earlier ones are 85mph speedos, only some of the 89 models had 140 speedos.