Swapping for an OD 904?

The overdrive 904 you're talking about is the A-500. And that thing is a fat dinosaur, and it does not simply bolt in. A518 is even worse.
http://www.forabodiesonly.com/mopar/showthread.php?t=106214
http://www.forabodiesonly.com/mopar/showthread.php?t=261064
https://www.google.com/#q=mopar+a+body+overdrive+site:www.forabodiesonly.com&safe=off