I have a 70 Chevelle 350 2bbl auto. I am trying to figure out if I have a THM350 or THM400. The engine code section of this site does not list an engine code of "CNN". I have located this at another site but they are saying a CNN comes with a THM350. My build sheet has a "M40" on it, which indicates a THM 400. Can anyone help me figure this out? I heard that only THM400 were used on big blocks. Any hints? Thanks in advance!! (Right now, I am going with my build sheet, THM400)