1969 GTS Drivetrain into 1964 Dart Question

Definitely do-able, but here are some obstacles:
1.) Can't use the '69 K frame, motor mounts/brackets, centerlink, sway bar, (front suspension is a different width- '64 is narrower), idler arm, clutch pedals, z bar is different, probably more. The '64 trans. crossmember needs to be modified to accommodate dual exhaust.
2.) The '69's exhaust manifolds MAY work, provided the '64 is manual steering; There may be some others that will fit with work, you'll have to do a search on here. Otherwise you're looking at custom headers, since I'm not aware of any currently being made for an early A big block most likely will have to be fenderwell exits so you will need to cut your inner fenders- and depending on how they're routed it could affect your steering; all those tubes in the wheel well limit your turning radius. You will need the early A four speed transmission floor hump. You will need the early A drop center link for V8s if there's not already one on the car. The brakes and suspension (Tbars, upper and lower control arms, spindles) will all bolt up to your '64 K frame. The rear suspension and 8 3/4 will bolt in. If your '64 was a slant 6, you'll need the V8 throttle linkage from an early A, although the '69's could be made to work. The '69 radiator *should* fit, although big block cooling in an A body is always dicey- and the much more restrictive early engine bay will only make things worse. There's undoubtably more.
3.) I believe modified clutch linkage is available from Brewer's. Exhaust is all fabbed. Have to get a driveshaft made or possibly the '69 one shortened. Swap motor mounts are made by Schumacher, although not currently available; HOWEVER, A body big blocks had modified mounting bosses, so the Schumachers may not work anyway. You'll have to fab something up.
Like I said, it's do-able; but not a bolt in. It'll take some work, but it's been done many times. Personally, I would find a later model A body ('67 and up) roller and bolt EVERYTHING in, or resell (lots of those parts tough to find because they are unique to the factory BB A bodies and worth beaucoup bucks).