71 Dart Slant Six w 904 into 64 Valiant

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
Exactly. Who might know how compatable the newer column and linkage would be. Ya, it might be better to just get the engine. What about the newer crankshaft and the older converter snout? That's an issue on the v8's.