I have a 1978 Camaro RS with a 305 engine. Everything appears to be original to the car, Intake, Carb, Exhaust Manifolds, Block casting number, etc. The block pad has the vin number stamped as well as the correct assembly date. However the 3-digit engine code is CEM and I am told that this code is for a 1980 305 block. Any ideas or speculation?