5th Ave spindles or any other FMJ body are supposed to work on those cars with some minor tweaking. Add a repop 75-79 Cordoba caliper bracket and you can fit the 11.75 B body rotor.
ECI makes a nice kit that is well engineered. Not cheap but reasonable given what you are getting. It uses your factory spindles with custom hubs and 1 piece caliper brackets and GM buy them anywhere calipers.
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When I learned that C-body calipers dried up at the parts store I started looking at some other parts also. (C-rotors still seem to be readily available, both 69-72 and 73 styles are reasonably priced.)
Looked at B-body calipers (seemingly good supply) but those require a thinner 1" rotor so won't fit on a C unless the rotor is changed.
The 11.75 B-rotor uses a different inner wheel bearing, so it's not a drop-in.
But what I saw was that the 11.75 B-rotor might be drying up. It is used on 75-79 B-bodies and the 79-81 R-bodies - so not a huge pool of popularity to drive sales.
The reproduction Cordoba caliper brackets were out of stock also. With all of the current supply-chain issues, many companies are bringing overseas castings back home, and casting houses are swamped (of the 5-ish ones I've spoken to). That may or may not be the reason, though.
IMO, ECI seems to me to be the best of the 'mom-and-pop' type kits (vs Scarebird, AAJ). AAJ has limited info on their website to evaluate and Scarebird uses bearing spacers, which I am not convinced is a good long-term idea.