Will a 3.1L from a 1990 Cavalier Fit a 1997 Grand-Am with a 3.1L, I found a rebuilt 1990 rebuilt Cavalier engine and I have a sick 1997 Grand-Am with a sick 3.1L

I believe it will, just some of the sensors and possibly the intake might be different, depending on the fuel delivery system used on the motor. The blocks are the same, heads should be as well.

physically they are pretty much the same, but you will need the intake and FI system off the GA, as Skibum said.
GA sensors will hav to be used also.

Its not realy a hard swap, just a bunch of BS to get it swapped and running.

Good Luck!