Yeah, it's almost like either the engine sits a little too high or the flywheel is a little too small. Either way, you can check by removing the engine and pressing down on the box like you normally would. You should be able to fully depress the truck on the box and still have the bump box wheel spin freely in the chassis slot.
Your best bet is to just grind on it, check it, grind some more, check again, etc. etc. until it fits. I had to remove quite a lot of chassis material to get it to work on my OFNA Stainless box, I'd recommend using a Dremel with a high-speed cutter bit to start, then once you have it working clean up the rough edge with a grinding stone. If you use a hand file you
might get done by November