This is not an easy task as I understand. Definitely not plug and play or direct bolt up. Tranny will not work, and even if you get it to work, the LT1's PCM will go nuts and the engine will not work well it the PCM is not connected to an electronically controlled tranny. you will have to get a 94-96 4L60E transmission, and you will have to have the computer (PCM) and complete wiring harness for the LT1. You will also have to notch the frame rail to accommodate the A/C compressor. It also uses different motor mounts. This will cost many times the value of your car, and it will takes a very very long time. It is actually easier to swap a TBI 454 into a 91, than it is to put an LT1 in it.
Here is someone who did it's write up to give you an idea of the work involved.
It would take a lot of work. The engine will bolt up just fine, but if you want AC, the frame will have to be notched on the passenger side to make provisions for the AC compressor. You'll also have swap in a LT1 Sedan gas tank/fuel pump/sending unit, and run new fuel lines along the driver's side frame rail. Not to mention trying to get it wired up so it'll run properly and have all the guages work. I'm sure there is a bunch of other stuff that I'm forgetting too. It has been done before, and I'm not trying to discourage you, but do you really like the 91 body style that much? Reason I'm asking is because it would be cheaper and much easier in the long run to just buy a 94-96 Caprice sedan that already has the LT1 in it. Maybe even find a cheap one that has a bad engine, and then swap your wagon LT1 into it. That's just my opinion. Good luck :smile: