Anyone use B body brakes on the A's

Ought to be okay. Make sure your master cylinder is for disk brakes (one reservoir larger than the other) and that you have a disk brake proportioning valve. If your 74 Dart already has disk brakes, you have those requirements covered.

Given that the 11" brakes are larger than those originally on the Dart, you might want to keep an eye on brake balance. If you find the rears consistently locking up first, you would do well to find an adjustable proportioning valve to redistribute more braking force to the front wheels.

If one axle or the other has to lock, it's best to lock the front. The car will slide in the direction it is pointed. If the rears lock first, the backend is inclined to want to pass the front.