Just judging from the rocker cover, I'd say it's a '58-'62. All of the '55-'57 235/261 engines I've seen have the filler at the front of the cover, the '58 and later seem to be all to the center. Looks like the radiator has been changed, too. The upper hose should be lined up straight with the neck on the engine, not at an angle.
In addition to the # by the distributor, there's also a casting date above the starter that will narrow things down as well.
For the rear gears, you'll have 5.13s. That's the only gear ratio that was used on those trucks, unless the center section was changed to a 3/4 ton, then it's most likely a 4.56. It should have a 4 speed, but there was a somewhat rare option of a heavy duty 3 speed in the 1 ton.

Bill Burmeister