It's entirely possible that the engine could be the larger 261 cubic inch type. They're difficult for the casual observer to tell from a 235 unless they're are aware of a few noticeable differences. The 261 was mostly used in commercial vehicles, of which the postal van would definitely qualify.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!