B body rear brakes onto A body axle
Uh, not necessarily on a Duster. It'll depend on what backspace you use.
Even the A-body 8 3/4 puts the wheels closer to the springs than the quarters, this will be especially true with the SBP axles/brakes (BBP adds about a 1/4" to 5/16" to the track width). Although it is wider than the BBP 7.25/8.25 rear axles by about an inch, it still isn't enough to center things up with factory wheels.
Even with 15" rims you'd want zero offset in the back to center things up with the widest tire you can run (275), while in the front you'd want +6 to +12 for the offset to maximize your tire sizes. And if you're not worried about maximizing your tire size, but you don't want the rear to look narrower than the front, you actually would need a negative offset in the back to push the wheels out toward the quarters. When I was running cop rims on my Duster (15x7, 4.25" backspace, 225/60/15) the rear looked absolutely goofy until I swapped in a B rear and kept the same wheels.
I run a 68-70 B body rear in my Duster with the perches relocated. Probably the exact same rear you had in your car (55" flange to flange). Using 17" or 18" rims it's very close to making the offset the same front and rear. A 65-67 B body rear is actually about perfect to match the front offset with the rear on a Duster/Demon/Dart Sport if you're trying to maximize tire width front and rear.
This is what I'm talking about. This is with the cop rims I mentioned, and 225/60/15 tires. This is a BBP 7.25, so it is narrower than an A-body 8 3/4, but an 8 3/4 would only move the wheels out about a 1/2" per side. You can clearly see that there's several inches to the quarter, while the wheels are tight on the springs. That's with a +6 offset on a 15x7 (4.25" backspace)
Now, I don't know what rims/tires you're planning on running. If you're staying with 15" rims the A-body 8 3/4 isn't as hard to deal with, there are still plenty of 15" rims available in 0 or negative offsets. If you're moving up to 17's/18's, having the wider rear is a much better match to the available offsets.
I will say that I wouldn't mess with the SBP brakes. Replacement parts are not always easy to find, and selection is limited. The drums themselves have fallen out of production several times, although they are currently being reproduced. And of course, any time you need new drums you'll have to have them re-drilled for the BBP if that's what you've done with the axles. Makes more sense to buy the aftermarket BBP axles and use BBP brakes, because they came on darn near everything from the mid seventies all the way up to the late eighties/early 90's.
Uh, are you really running C-body brakes?
Meaning, you changed out your spindles to C-body spindles and re-drilled them because the lower ball joint spacing is different between A and C bodies? And you're running 1973 C-body spindles, because those are the only ones that use the same upper ball joint diameter? And of course they're 5/8" taller too.
Or are you running the later B/R body 11.75" brakes which bolt right on to 73+ A, and F/M/J spindles?
Uh, not necessarily on a Duster. It'll depend on what backspace you use.
Even the A-body 8 3/4 puts the wheels closer to the springs than the quarters, this will be especially true with the SBP axles/brakes (BBP adds about a 1/4" to 5/16" to the track width). Although it is wider than the BBP 7.25/8.25 rear axles by about an inch, it still isn't enough to center things up with factory wheels.
Even with 15" rims you'd want zero offset in the back to center things up with the widest tire you can run (275), while in the front you'd want +6 to +12 for the offset to maximize your tire sizes. And if you're not worried about maximizing your tire size, but you don't want the rear to look narrower than the front, you actually would need a negative offset in the back to push the wheels out toward the quarters. When I was running cop rims on my Duster (15x7, 4.25" backspace, 225/60/15) the rear looked absolutely goofy until I swapped in a B rear and kept the same wheels.
I run a 68-70 B body rear in my Duster with the perches relocated. Probably the exact same rear you had in your car (55" flange to flange). Using 17" or 18" rims it's very close to making the offset the same front and rear. A 65-67 B body rear is actually about perfect to match the front offset with the rear on a Duster/Demon/Dart Sport if you're trying to maximize tire width front and rear.
This is what I'm talking about. This is with the cop rims I mentioned, and 225/60/15 tires. This is a BBP 7.25, so it is narrower than an A-body 8 3/4, but an 8 3/4 would only move the wheels out about a 1/2" per side. You can clearly see that there's several inches to the quarter, while the wheels are tight on the springs. That's with a +6 offset on a 15x7 (4.25" backspace)
Now, I don't know what rims/tires you're planning on running. If you're staying with 15" rims the A-body 8 3/4 isn't as hard to deal with, there are still plenty of 15" rims available in 0 or negative offsets. If you're moving up to 17's/18's, having the wider rear is a much better match to the available offsets.
I will say that I wouldn't mess with the SBP brakes. Replacement parts are not always easy to find, and selection is limited. The drums themselves have fallen out of production several times, although they are currently being reproduced. And of course, any time you need new drums you'll have to have them re-drilled for the BBP if that's what you've done with the axles. Makes more sense to buy the aftermarket BBP axles and use BBP brakes, because they came on darn near everything from the mid seventies all the way up to the late eighties/early 90's.
Uh, are you really running C-body brakes?
Meaning, you changed out your spindles to C-body spindles and re-drilled them because the lower ball joint spacing is different between A and C bodies? And you're running 1973 C-body spindles, because those are the only ones that use the same upper ball joint diameter? And of course they're 5/8" taller too.
Or are you running the later B/R body 11.75" brakes which bolt right on to 73+ A, and F/M/J spindles?