i found an rx7 rear end from a 86-91 car, with a supposed 3.9 and they have it labeled as slip diff, would i be safe to assume that it is the clutch type inside the rear end?

 

The diff should have a metal tag on it which I was told means it is a LSD. The GSL models all had clutch type LSD within those years. For it to have a 3.9 i think it had to be a 84-85 though i could be mistaken.
To be absolute sure, get the vin and call mazda. They can usually tell you exactly what the car had.

 

i know the 84-85 gsl had a solid rear end, and the one I'm looking at is not. I've sent him an email regarding whether it was a non turbo, and if it had a torsen or clutch type lsd, and the only answer i got was that it was non turbo, so i shot him another one, asking him what it was.

 

The automatic RX-7's had a 3.9 rear end. The turbo rear end is a completely different diff in itself.

 

I do know the 84-85 GSL is a 3.909 and the 84-85 GSL-SE is 4.08.