I asked Jeff Helms about the IY code listed in Alan Colvin's book and why it listed the engine as a 396/375 HP motor.
From Jeff: "When Alan wrote the book he did not have the COPO sheets from Tonawanda yet that explained the engine was a 396/425 and how to build it, so it remains correct as a 396/425 motor. Someone out there has the specifics on the COPO build, I'm sure we'd all love to see
Also, perhaps someone has information on the engine past number listed; 3888819 or the other COPO motor listed above this one, COPO 9557: 3882847?
Paul