Pretty much all diesel engines use one of four SAE bell housing patterns, the trick is to find the intermediate section that matches the SAE pattern to a Ford, Chevy or Mopar bell housing. The most common is available out of old bread trucks. They had various diesels that mated to 700R4 transmisions. so they GM transmision friendly.
Check out the small diesel engines from Kohler, they sell a couple small ones that I could see dragging a metro or sprint body around in the 50+ mpg range quite easily. If they were Highway rated... Which they have to be where I live...
Dave