can you stick in an sa gauge cluster into a gsl-se? or does it require modification?

 

They aren't even close to being the same. Different physical shape and different wiring. Any 84/85 cluster will be a direct replacement to the GSL-SE cluster.

Are you just wanting to replace a broken cluster, or are you trying to get the look of the SA cluster? If you really want to use an SA cluster, it can be done, but it will require some work.