I'm considering an A 500 4 speed overdrive for a 72 Duster

RustyRatRod said

Then you should consider the GM 200 4R swap. There are a few threads on it as well. Little to no floorpan work.

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No floorpan/trans tunnel work involved with putting a 200-4R in my '69 Dart.
The gear spread is 2.74/1.57/1.00/0.67. I'm going to be running 4.30 gears out back, but it'll be like having 2.88s when in overdrive.