71 Dart Slant Six w 904 into 64 Valiant

I would buy the 6 and the 904 that comes with it. Get the slip yoke driveshaft if you can.
Even if you use the 71 engine and trans with the 71 driveshaft, there is still the shifter situation. 64 has a PB cable shifter, the 71 trans has linkage type shifter, so will need an aftermarket shifter.

Will also need to use the 1964 carb, or convert to cable throttle linkage