cost of a diesel

[mr48chev]

A photo works good in showing us what the problem is. If the case is junk you can probably find a good case pretty cheap get a seal kit and swap the insides of your trans into the other case or have someone do it.

Outside of a 6.5 you are looking at several thousand dollars for a Cummins 6 or even more for a Duramax plus the trans plus all of the extras.
The 4BT's out of the bread trucks work good but most of them are pretty well shot by the time they go on the market and have to be completely redone. You might want to look at this site as they are full of info on swapping them http://www.4btswaps.com/forum/forum.php

I'd price things out before I ever went hunting for an engine and figure out if what savings you might get would come close to matching the expense of doing the swap. Spending 10 K on a swap to save 500 a year on fuel might not be the most prudent thing to do in the long run.