First of all I would like to congratulate you on a fine engine, Jerry. You don't say whether you have just the engine or if you have the whole truck, but I assume just the engine.
The 235 engine in this year for the 3600 series is a light duty truck engine and was originally painted gray. It should have a decal on the rocker cover that states "Thriftmaster 235". This was the same for the 3100, 3600, 3800 and 4000 series engines.
This is as opposed to the 5000 and 6000 series and the forward control models, which have a "Loadmaster 235" decal. This engine was also an option for the 4000 series. It was also painted gray.
The optional engine for the 5000 and 6000 series in the 235 was painted green and had a decal on the rocker cover that said "Jobmaster 261".
My father had one of these trucks when I was younger, although it had been restored by someone else before he purchased it. The engine had been painted black by that person, but after doing research, my father found the information above and shared it with me.
It was a great truck and a classic. I hope this information has helped you.