74 to 70 Dart parts swap

I'd do the entire K frame swap to the earlier dart. Undo the upper control arms and drop it all out.

A couple of large furniture dollies from Harbor Freight with some plywood on top work great for rolling the engine/trans around.

That's an 8.25 rear. Check/replace the fluid, clean it up and install it in the 70. I hope you don't like the wheels (small bolt pattern) on your 70, they won't work on the 74 rear/disc (BBP).