Pushbutton to 4 speed 64 Dart

Will your crank accept the 4-speed input shaft; that is the big question?
There is a work-around for this if it doesn't.

You got bucket-seats?
If yes.; personally, having already done this in a 68 Barracuda; I recommend a long-tail trans and move the shifter back between the buckets and up as high up as you can. Say goodbye to missed shifts. My shifter is up high enough that the upper pivot bolt is above the floor line. And I use an old Mr.Gasket Bang! shifter, with it's very short stick. And you gotta use a ball for a **** knob, you just gotta. There are a thousand hand-positions you can use, to fit every conceivable situation.
But yeah, you are gonna need some fabrication skills.
But the PB automatic is just a PB automatic. It rebuilds just like any other 904.