Barn find - floor shift - what did I find?

Hi all, here's a 73 Dart Sport that came with a 318 and a manual trans... it has what looks like a big rear end under it. Back seat folds. Plenty of patina! No engine or trans. Can somebody tell me more from the tag or photos? What's that rear?

I do appreciate it! Long time lurker, I love this place.

~Scott in TX