Anybody know if the PS gearbox from 65 Coronet fits a 65 Dart?

I would think it has a large sector shaft. You might have a problem getting a pitman arm for it for a early A.
Nope, it has the small shaft just like the A-bodies. Depending on year and engine size, the only differences are the hose fittings on the control valve. Those can be swapped over from the old box.