what are my realistic manual transmission swap options?

To get driving, I would rebuild the 904 (fairly simple) or install another (known-good) slant 904. They are cheap to free. Do the V-8 swap once you have everything (including new rear, disk brakes, wheels, ...). Mopar 4-spds are rare, expensive, and heavy so maybe consider a more modern T5. There are adapter plates ($$$$). But wait until you can afford the dream. The Dakota option above sounds smart. If it can move a truck fast, think what it would do in your little A.