The C4AE heads are 64-65 352/390 heads with 71.2-74cc chambers. The C6AE-R are 66-67 352 heads with the same size chambers as the C4's. Both used stock 1.55/2.02 valves. By spec they are not 428 heads but indeed can be used. Both are a very good head with some CJ size valves and some port work. Thr C6-R's are often called the poor mans CJ heads. They are a common head and can be had reasonably priced. You could install some CJ sized valves, port and be close to a set of CJ heads. The CJ chambers are a little bigger and also a different shape. Unless you need a number correct set of 428 heads for a resto either set would be fine but most seem to favor the C6-R's. What are the specs on the 428?

G.