What tranny's are all compatable with the Il6? And how common would they be to find?

Why would you want to put this in a car???

C4 for sure.
Little 3 speeds too that came stock with them, I dont remember the part number though.

I think that other 302 and 351W transmission will mount to the housing too but you have to check the input shaft lengths for manuals. For auto's you have to make sure that you have the proper flexplate and torque converter.