In this pic, you can see a stub axle coming off the side of the transmission, at the left side of the frame. In the donor, that (and the one on the other side) sent power to the front wheels. The driveshaft at the tail of the transmission, to the right of the frame, powered a drive shaft for the rear wheels.
I don't need that second part. Because I am relocating the transmission from the front of the donor car to the rear of the kit, what used to feed the front wheels will feed the rear wheels instead - and the rear drive shaft just won't exist.
In order to get this in, I took off the last segment of the transmission case, pulled out two geared shafts (I swear I thought I had pics) and a differential, and then installed that collar to lock the rear of the transmission.
This coming weekend, I am hoping to make progress of getting the engine and transmission mated, in prep for the first (of many, I am sure!) introductions to the frame.