Dave, I saw an unrestored 3/4 ton and the engine was olive. Not sure if you could call it olive drab, but olive green in color.
As far as paint codes I know of none.
I have a 39 GMC also, and bought a quart of engine paint from Jim Carter. However, it turned out to be more green and not so olive. I used it anyway and grown to like it.
Unless you'r restoring it all the way to orginal, paint it a color you think looks best with the overall paint scheme.
Jim
