I traded the old T-5 trans out of my Mustang for for a C4 out of a late '60s Mustang. I plan on bolting that up to a 5.0 and dropping it the YJ. I assume I can pull the tailshaft off and bolt up a t-case. Anything I need to look for (spline count, etc) on the C4, and are there any good t-cases with a driver's side drop that will bolt up to the C4? I heard a Dana 20 out of an early Bronco would work.

I dont know about the spline counts but the early broncos did come with the C4 and dana 20. It would probably be best and easiest to get a pair that is already matched together or at least a C4 from a 4X4. A 2wd C4 will most likely need to have the tailshaft swapped out for 4X4 version.

Advance Adaptor has an adaptor and new tailshaft kit for almost any xfer case. As long as you have the C4 apart consider putting in a lower low gear and heavy duty clutchs.