Early B&C-Body Brakes
According to Mike Martin (Mopar Suspensions, S-A Design Books, 1984, p. 66):
The A-body will accept various components from the B-body that will improve braking. First of all, the totally inadequate, 9-inch A-body brakes and spindles MUST be replaced with either 10-inch A-body pieces or, if the upper control arm is modified (to accept the larger ball joint used on the B-bodies), the B-body 10-inch, 11-inch, or disc spindles can be installed. In either case, the B-body backing plates and shoes will easily bolt on.
If the B-Body spindles (disc or drum) are installed, the lower control arm does not have to be modified to accept the larger B-body lower ball joint - the A-body tie-rod ends will also work without modification. The upper control arm can be modified to use the large disc brakes form the Road Runner and Charger R/T. However, if you prefer to use the 11-inch B-body drum brakes, the backing plates will not bolt onto the 10-inch spindles. In this case, the 11-inch B-body spindles must be installed along with a modified upper control arm.
The A-body spindle and lower ball joint are smaller than the B-body pieces. Because of these differences, both spindle and ball joint must be changed as an assembly. And since the caliper mounts are different, the B-body calipers must be used with the larger components.
The standard rotor and hub assembly used on 1965 to 1969 A- and B-body cars is the best choice for most disc brake applications. The long lugs [which also fasten the rotor to the hub] may interfere with some lug-nut lock sets, but they add an extra measure of insurance for high-performance and competition use.
[The book shows the photo of a] 1968 Bendix B-body disc brake assembly [that] has been installed on an early A-body. The massive caliper and 12-inch rotor (with a 4-1/2" bolt circle) make an enormous difference in braking, handling, and safety. Cool air, ducted from under the front bumper is directed at the caliper to aid cooling while road racing.
Brakes even larger than the 11" passenger-car brakes (such as those used on the C-body Fury or Monaco police cars) can be installed if the socket in the lower control arm is machined to accept the larger C-body lower ball joint.
Any modifications to the upper or lower control arms, ball joints, or sockets require precision machining. The upper control-arm bearing retainer must be cut out to install the larger ball joint and a new retainer carefully welded in and machined to size. When the lower control-arm socket requires reaming to accept the larger ball joint, the same precision machine requirements apply. These specialized modifications should be left to a machine shop with stock-car or road-racing experience. ...
Even though "Mopar Suspensions" was published in 1984, the information in the above passage seems to indicate that most of the A-body section was written in 1970 because Martin isn't aware that the Kelsey-Hayes A-Body disc brakes were produced until 1972 and that the 1973 to 1976 A-Body upper control arms obsolete his advice about modifying the upper control arm to take the B-body upper ball joint.
Although Martin mentions that B-Body and C-Body spindles are bigger than A-Body spindles, he doesn't mention anything about the mounting dimension differences, which implies that they can bolt-in without having any impact on handling. Richard Ehrenberg seems to confirm this in his Disc-O-Tech article. Because the 1974-1978 C-bodies used a different suspension design with unique C-body spindles, Martin must be referring to brakes used on 1966 to 1972 vehicles.
Because of the relative scarcity of these early brake components, the best braking option is to use the 1973 and later braking systems.