automatic overdrive

1..So far as I know, it's 904 based AKA 42RH series
2..GM 200R4 or 2004R whatever they are---genuine 4 speed, not OD, but hard to find and a bit weak
3...Gear vendors aftermarket, "not sure" if you have to cut the floor

The Mopar OD you must extensively rework the rear cross member.

And yeh. There ARE a lot of threads on this, and a couple with great photos showing the process and crossmember mods