Looking for an anto od trans for a 66 /6 Valiant

If you're looking for a bolt-in answer, it ain't happening.
The Mopar and GM overdrives will both require adapters or bellhousing surgery and replacement, both will require crossmember mods of varying degrees, and they'll both need modified or custom linkage and specific wiring to those transmissions. RH series Mopar ODs can be run stand-alone, RE series will require either a controller or a set of toggle switches wired in for controlling the functions. Mopar units will require floor pan/tunnel/crossmember mods, the GM will require a custom speedo cable, and *possibly* some tunnel mods in the tight confines of an early A tunnel.. Both will require a new custom driveshaft and a custom transmission mount.
Probably your best bet and the least amount of work would be a Gear Vendor's overdrive unit mounted to your existing 904. It will still require a new driveshaft and speedo cable, and POSSIBLY some tunnel mods, but still the least amount of work.
Dodge 3-Speed Auto - Gearvendors