If I remember this part of the kit correctly, the outside surface of the cowl panels are simply larger than the corresponding opening. I think they're just plain a
bit too big.
Remember, this is a 60's kit, and operating features on static models were at the height of their popularity at that time. The kit has a "fully detailed" engine and can be exposed from both sides. Perhaps the slightly large cowl panels were engineered purposely to ease the removal of same.
And the gaps in the wings aren't caused by any inverse transference because of an uneven mating surface. You can actually see daylight through these gaps, lots of it. (
This I remember very well, as opposed to the rest of the kit!)
It's just a plain bad fit. You could stretch the fuselage width at the wingroots with an internal spreader, but that would put strain on the fuselage seams and throw the alignment of the mating surfaces way off. The only viable choice, again, is to fill it with plastic stock, strip, card or whatever you wanna use.
Despite it's good points, it's really, overall, a kinda ugly kit. And it pains me to say that, 'cuz I'm Mr.P-40!
Fade to Black...