1971 Duster-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.
To drive at walking speed with a manual trans and cruiser gears is as good as impossible. If there will be slow driving involved like parading, then I highly recommend to stay with an automatic.
If yur gonna stick with the automatic;
@oldkimmer has you covered.
However, the Mopar 4-Speed Automatics, with their overdrives are a great option, but you will need to modify stuff to get one installed; and they run real good with 3.91s! But for a cruiser, 3.55s would be nice, at 65= under 2000rpm. This is a really hard combo to beat, for any usage.
As for cruiser gears, for autos; your choices are from 2.76 > 2.94 > 3.23, Pick one according to your cruise speed and comfort level. Personally, I like something closer to 2.45s but the performance takes a real dump with just a 3speed auto.
As for Mopar M/Ts
For a cruiser,
the A833od might be a good option for you. With a stock-cammed 360, tuned to run at 500rpm all day, and to get to 3.5 mph walking speed with 27" tires, this would require 3.73s.
And that 2.73combo would cruize at 65=2200 which IMO, is fabulous, cuz you can optimize the cruise timing to run that, and get the best economy out of your 360, that she can give..
However;
The gear-splits, especially the 1-2, are pretty wide, but the lo-po hi-torque stock 360 will easily pull them, and Second Roadgear of 6.23 is actually pretty decent..
If you're into parading and just have to have an m/t,
IMO, this would be a good way to go.
BTW-1
I have, in the past, run this A833od, mostly with 3.55s, for 4 years, and altho my engine was a lil too hot for it, it put up a good fight. In the end tho, I swapped it out for a Commando A833.
BTW-2
I might be able to help you with a conversion.
But I suggest to give oldkimmer's advice a try first.