74 dart drum sbp to lbp disk

Hello there,
The Diplomat/Gran Fury/5th Avenue cars all use the same knuckle/spindle and will bolt into your car just fine. The Aspen/Volare cars are the same as well. You will need lower ball joints for a 73-76 Dart with disc brakes. If your car was equipped with the 10" front drums, you are good to go. If the front drums are the 9" size, you will then also need to get different UPPER control arms. The disc and 10" drum cars used the UCA with the larger upper ball joint, which was the same one used in the B and E body cars.
Be careful about how you route the brake hoses! The factory arrangement for a 73-76 Disc a body is with the calipers mounted forward of the hub. The donor cars you mentioned mount them to the rear. The rear location often results in the hoses bunching up in a loop, and they can rub the tires and wear through eventually.
Greg