An A-body 8 3/4 will bolt into any A body from 1963 through 1972. In fact, it will bolt into later A bodies, too, but will have a small bolt pattern vs. the 73-later large bolt pattern. (And I believe an A-body 8 3/4 would also fit pre-63 A bodies, but haven't tried that combination.)
Rear springs are interchangeable 1963 through the end in 1976. (They also fit 1962 Dodge and Plymouth B bodies, whose springs have the same 20-inch front segment.)
If you now have a 7 1/4 rear, the original driveshaft will have to be shortened. Or you can use the driveshaft that goes with the 8 3/4 so long as the donor 8 3/4 is from a Dart and the donor's transmission is the same. A 727 is longer than a 904, and so would use a shorter driveshaft. If memory serves, the 3 and 4 speed manuals are the same length as a 904, but most 4 speeds have a larger main shaft than 3 speed manuals or 904s, so the front yoke would not be the same.
I have the rear end and driveshaft from a 71 Duster 340 in my 65 Barracuda. Because the Duster had a 3-speed manual vs. the Barracuda's 4-speed from a 69 Dart, I did have to change the front yoke. The driveshaft fit perfectly, though. Early Barracuda/Valiant A bodies had a shorter wheelbase than later ones (106 vs 108), but the engine/trans sat in the same place relative to the rear end. A Dart, of course, has a longer wheelbase than the Valiant/Barracuda, so the Valiant/Barracuda's driveshafts would not fit in a Dart and vice versa. (The exceptions being the Dart-based Valiant Scamps, which had the 111 inch Dart wheelbase, and the Duster-based Dodge Demons and Dart Sports, which had the 67-later Plymouth A body 108 inch wheelbase.)
Also: what Professor Fate says. Shock plates for a 7 1/4 won't fit an 8 3/4, and the 8 3/4 also has larger axle tubes than a 7 1/4, so the U bolts are different, too.
