The 221 c.i. V-8 has 24 studs per head. The smaller V8-60 has ?17? per head. I'm pretty sure though, that the smaller V-8 was not installed in '40 trucks, rather, the 4-cylinder (tractor) engine was used.
Of note: 39-41 BIG trucks had a 239 c.i. engine, designated the 99T. It has a slightly raised surface for the intake manifold.
It's very possible that your truck does not have the original engine, in which case, the only sure way to determine the displacement is to measure the bore and stroke.
I'm VERY sure the engine is original, mainly because it was originally purchased from a person who I believe bought it new, and just kept it around his place for a good few years. now its time to restore it, and clean things up a bit. would the color of the block be anything of a clue?