So I decide to get to the bottom of what's wrong with my swindle; the results weren't good or conclusive...
I shaved some plastic off of where his legs overlap his - um - crotch bulge in vehicle mode, that helps a bit, but the issue seems to be that parts unknown seem to have been miss-measured somehow.
When the windscreen is removed from the rocker arm, it sits forward of the rocker arm by a few millimeters and doesn't line up with the arms, but I can't get past the fact that his leg shells don't line up at all with his crotch bulge, and I think that's very telling of what's wrong with him.
And yet, if they were shortened to line up, the leg would be shortened to be a different length than all of the other combine limbs, so my only conclusion is that some part of the leg were discovered to be the wrong length at some point, and were hurriedly remolded with nary a thought of what the consequences could mean