1991 Pontiac Grand Prix replacing motor

You can call a junk yard for interchangeability information.

But you are going from an OBDI to and OBDII, so most likely you will have to change the entire wiring harness in car.

You would be better off finding out what years are interchangeable with the 1991 and use on of those engines.

The only other option would be to strip the '96 down to long block and put everything from 91 on 96, but you still may run into problems.