The C6AE-U heads were one of at least 6 common FE cylinder head castings used in the 1966 model year, the others being the C6TE-G, C6AE-J, C6AE-L, C6AE-Y & C6AE-R. The C6AE-U heads came in both 8 exhaust bolt pattern for the full size cars, T-Birds & pickups, and 14 exhaust bolt hole pattern for cars with shock towers. They also came with the the Thermactor ports drilled or undrilled. From what I have seen since I’ve been logging casting dates is the various 1966 model year common cylinder heads tend to run in date ranges, and some overlap or run concurrently with others. The C6AE-U heads by casting date run toward the later half of the 1966 model year, and overlap with the dates of the C6AE-R heads. The C6AE-R heads, just by what I’ve seen with the casting dates, seems to have had the longest production run during the 1966 model year, followed by the C6AE-U. Ford seemed to be using most of these heads castings that model year interchangeably, and there are multiple reports of original cars that have one cylinder head casting on one side and a different one on the other side, of the head castings that were being manufactured at the same time period. Performance wise the C6AE-R heads are probably a little better, with the taller intake port, but remember that as cast the C6AE-R heads have that combination exhaust port with both the floor of the port having the earlier FE head higher location, and the roof of the exhaust port having the lip of the lower location of the later regular FE heads, so they tend to have the smallest exhaust port exit of most FE heads, unless you remove that lip. The C6AE-U heads are probably closer to the C7AE-A heads.
