Good luck finding the 3.1 turbo.

Just go with the L67 swap with the 4t65e-HD. It's a more powerful engine compared to the 3.1 turbo anyways, with much more aftermarket parts. I heard the L67s fit right in without any modding, maybe very little. However, the 89 Grand Prixs are not the same as the 94-96..though I don't know if that matters for mounting the engine and tranny in.

www.w-body.com is a good place to research this stuff.