If I did the correct research - I've answered my own question, but my answer doesn't offer any explanation for usually high engine RPMs (for a diesel) after a 5 speed swap.
The 4th gear (overdrive) in a ZF Automatic should be 0.73
The 5th gear (OD) in a BA 10/5 should be 0.79.
The BA 10/5 info came from a Jeep website - as AMC used that trans behind the 256 cu in Inline 6 from 87 to 89. (And the Jeep guys hate it BTW - so they might be a source for cheap BA 10/5s) if all the splines and everything match up to the 505... seems doubtful.
So the ZF Auto would give you a tad lower RPMs at 70MPH but not a lot lower. I did see a post about rear end gearing that said the Wagons have a 4.22/1 rear gear in them vs the Sedans with 4.11/1 gear - so the Wagons would turn a bit more RPM on the highway, but that would be true with either the ZF Auto or the 5 Speed.
Anybody know what RPM a 5 speed Turbo Diesel wagon turns at 70MPH - assuming stock wheels & tires?