If it is a "true CJ block" it will have the sand scratch "C" on the back left hand side of the block. Many FE and FT blocks have the extra webbing on the cranksaddle. Go to
www.428cobrajet.org. This is the cobra jet registry. They have a ID section that covers ALL parts on the CJ/SCJ motors with pictures and correct part numbers. It might be a 390 punched out to a 4.13 bore (std 428) or the guy is full of S#%T or does not know what he has. G.