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

Even the RH/ RE Mopar ones require extensive rear crossmember surgery and may require a bit of floor pan mod

The only other option I know of MIGHT be a "Gear Venders" which is made to bolt in place of SOME Mopar tail housings. You will have to research that. So for that you'd need a factory Mopar auto and YOU MAY need to update it to a later model to fit the gear venders, don't know. IN ADDITION the early cars had small snouts on the torque converters meaning the pilot hole in the crank is also small.

I would ditch the idea. People drove slant powered cars for decades with no OD