Others will chime in with better interpretations:

D = 1946 regular engine ( 216 on lighter trucks; 235 on COEs; or, is RP0 227 needed for any 235 that year ?)
D = series designation ( 5100, 5400, 5700 COE )
A = Flint, MI

???

Post the casting codes from the engine and head. Someone might be able to help you identify it. I think that a 235 was standard on a COE that year. However, the engine might have been changed a few times over the years.