So, you're going to BBP and you have '73+ disk spindles. I just don't see any reason why you don't use the 73+ calipers. You can buy the brackets and everything will bolt on. Sure, the fixed caliper, multi-piston set up should be "better" in theory, but, remember that you're not talking about a modern multi-piston set up. The old factory multi-piston calipers have their issues with seals, leaks, etc. If I was going to go through the trouble of making a custom caliper bracket, I wouldn't be doing it to use old factory calipers, I'd make it to use a new caliper. There's lots out there, mustang calipers like Dr. Diff uses for his larger disk kits (like the 13" kit I have), Lexus SC400 like in this thread
Roll yer own bigger brakes (maybe), that kind of thing. I wouldn't do it to run 1960's tech calipers.
As for the B-body spindles, yes, I think you can use them. They should be the same height as the FMJ spindles, so, there would be some geometry changes but you can read about that here.
Swapping Disc-Brake Spindles - Mopar Muscle Magazine Personally I actually like the FMJ spindles better than stock A-body spindles. They also use the same lower ball joint as the 73+ disk cars, so, that shouldn't be an issue. To use that spindle with the later 73+ rotors though you would need a set of bearing sleeves, because the 66-69 B body stuff has a smaller inner diameter on the inner wheel bearing. Should be the same as the 70-72 B/E body stuff, so, you could get the sleeves from Dr. Diff.
'70 - '72 Mopar B/E-Body Disc Brake Spindle Sleaves. That's just going off the wheel bearing specs in the book though, I haven't done it. Otherwise I don't know what you would have to use for a rotor. I know the 70-72 B/E body stuff used a 2 piece rotor and you can get a reproduction version of those, not sure about the 66-69. And if you wanted to run the 11.75" rotors you would need the sleeves and a caliper bracket solution again.
