A body overdrive transmission

You gotta cut the crossmember out of the way for either one, and shorten the driveshaft. The 518 is a bit larger and longer. The USCarTool rear transmission support is supposed to be strong enough to take the place of the cut upper hoop, but I bent two pieces of 1/8" flat steel into half circles, one inside the other, plug and edge welded them together, clearanced the floor a little, and re-joined the crossmember together.
Driveshaft angles will change some- a little more with the 518 because of being longer, but enough to change the pinion angle will be dependent on where it was in the first place. I was moving perches anyway, so I used the Tremec app and got it where I wanted it.
Throttle pressure/kickdown mechanism still needs to be used unless going to a full manual valve body. You need a longer speedo cable (I still haven't gotten that to work, I think I need to "message" the floor/tunnel because of interference.
42RH / A-500 in a Duster with pictures.