406 was a 62.5-63.5 engine option only. I know the C4AE-A 427 block was used as a 406 service block casting. But never heard of a C5AE version. Is there any other numbers or letters after the C5AE? You might see a 6015 then another letter. The 6015 is just the basic engine number but the letter after that might help with ID. The 428 came out in 66 and I'm sure there could be some very early castings with a C5 number. But I've never seen one before myself. The thing with the original 62-63 406's were they were solid lifter only blocks with
oil pressure relief valve above the rear cam plug among other things. And the 427 top oilers had the same features. A true hipo block. Are you positive it's a C5AE number? If not do you see a HP anywhere on the side of the block? Pictures are worth a thousand words here if you can post some.