Wow, Surprised nobody responded on this one yet. I haven't swapped one from an manual to a GM trans but, I have done one from a manual to a C4 auto. You should be able to use an aftermarket shifter made for a C4 trans. B&M make a nice one that should mount correctly and the cable has plenty of adjustment so you shouldn't have any trouble getting the gears to line up with the shifter.
So what did you do with the trans you pulled out? I am looking for a T5. One that works would be great but I would also take one that needs to be rebuilt. I am swapping a 90 LX 5.0 auto to a T5. I would be interested in the flywheel, bell housing, fork, trans, and shifter. . I have the pedals and cableSend me a PM if you have it still and would want to sell it.