I have a 2wd 4r70w transmission, what additional parts will I need for this adapter? Or what is the easiest/most economical path to convert and adapt to a D20?

The adapter is complete. You will need to modify the crossmember, have a way to shift/control it, and have driveshaft modifications to think about. With the 2wd 4R70, this is the easiest most economical path.