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

The A500 is the same length as the A518.
Also, the 500 requires the same modifications as a 518, as it's not the transmission housing itself that requires mods to the tunnel, but the bulky flange at the OD-section, which is the same on both transmissions.
I'm currently rebuilding an A500 for my '73 Dart and installing a TransGo Reprogramming kit at the same time.
I currently have an A518 in the car and it eats horsepower.