1971 Duster-transmission

I have a '71 Duster with a three speed auto tansmission. Car started life as a Duster slant 6 but was modified with a 360 CI engine at some point. Current tansmission is really rough. Slams into reverse and doesn't shift well. I would like to replace it with either a 3 or 4 speed manual or auto transmission.

I need advice on this as to which type tranny and perhaps manufacturers and what gear ratio in the differential would work best with the chosen tranny. This is a cruise car only, no racing.

Thanks!