Test fitting and mockups done many times before even thinking about paint seems to be the norm anymore. While the detailing that the kit makers are putting into kits is improving with most new issues these details also make the different parts hard to get to align with one another often. Your mention of issues with the interior fitting on your latest build and I have shared the same problem as well. When test fitting the interior also look at the area where the windshield and rear window would mount. One area that seems to happen most often has to do with the width of the area where the dash in mounted in the interior and where that will set inside the body. The windshield will sometimes extend below the point where the top of the dash meets the underside of the lower windshield molding, and the windshield will prevent the interior from mounting high enough into the body and in turn does not allow the chassis to mount inside of the body correctly. County to ten and taking a break can also help.