Suggestions welcomed

Another option besides the expensive gear vendors OD bolt on unit? SURE! But you still may not like all that is involved, so...

The Chrysler A-500. It is a 904 with an overdrive unit on the back end of it. It requires some cutting and minor hammering from what Ihave read here by the "Who has done it" on this forum. Do a search, you'll find it.

The GM 4 spd auto transmissions are also used by some. Also expensive with adapter plates for trans to engine.
Also done by a few on this site. Again, do a search and you'll find it.