Anyone install three speed with overdrive behind a slant six

Plenty have done it. It's not a "three speed with over drive". It's a Mopar A833 four speed over drive. It has the A833 designation. That's four speed, over drive or not.

Four speed automatics are not called "three speed with over drive". They are called "four speed automatic" because that's what they are.

But yes, with the right gear ratio, it can be a good swap. I would go somewhere between a 3.73-4.10. That will take advantage of the over drive and also make the gear jumps better to over come. The final drive ratio for the over drive transmission with a 4.10 for example is 2.99. So don't be afraid of using a lower gear ratio. Thay are a better match for the over drive transmission, IMO.