[irox]

The ".707 top 3.9 diff" is what I've been looking for.
Although I would swap the 0.64 gears from the auto transmission (apparently you can do this without dismantling the entire transmission - and can even do it without removing the transmission if it's already mounted).

This should give 2392rpms at 80mph.

There was a subaru trans for their diesel engines with a 0.56 top, it's on 4.444 diff, but still delivers low RPMs compared to other offerings.

I also wonder about a custom trans, with the 3.545 diffs swapped into a 3.9 diff transmission. But not sure that would be possible without significant fabrication...

Cheers!
Ian.