Duane,
Even some 390 blocks will have the 66/427 casting numbers on the back of the block. This was done by the foundry to lower costs on the core changes for the different castings. These numbers by no means mean anything. What is important are the block casting codes under the passenger side #1 cylinder, bore size, and date code of the block located in front of the
oil filter adapter surface. Many 390 and 428 blocks have even the same block casting codes, so the mystery continues. This is why we sonic test every block that comes into are shop to verify what block we are dealing with. If anyone has any doubt, please call me to get more info. Thanks Much, Tom Lucas FE Specialties
916-339-0427 e-mail
feguy@msn.com