The block is probably numbers matching but what cam is in it? I bet someone did a cam change and head work. I had 2 1970 340's and both had stamped rockers, the T/A's heads are a little different. It's no big deal, they work fine but I don't think it rolled out of the factory that way. By the way, my 1970 Duster had U heads. It was a late production model and my 1968 Fury 3 had rear drums from a 1969. The shoes are different, it drove the mechanic replacing them nuts.
