I've done this figure a few times and have a current wip thread on it. Yeah, that particular spot right there doesn't like to fit tightly. HOWEVER, I just pushed really hard on it and it snapped into place. I think it has more to do with how well you transform the internal parts of it like the arms and all that are packed inside for car mode. That part WILL go down, but like I said, it's kind of a pain in the ass. By the way, what preparation work are you needing to do? Mine didn't require any sanding at all whatsoever and my current one is a full repaint.
Also, those flexible pipes on his hips need to FIRST be sprayed with acrylic paint to seal them OR mask them off really well because if you just prime them or base coat with enamels or lacquer, then they'll get all tacky and nasty. Plus, the head LED is super, super easy to swap out for a blue one. The head comes apart real simple and goes back real simple and once the head is off, it's just a matter of cutting out the LED and swapping it for a new one because it'll sit right on top of the neck base all out in the open and all
One more thing, I didn't bother taking apart his gun arm because that whole fricken arm is all wired up and loaded with gearing and small springs that will make you hate your life if you have to figure out putting it all back together just right after opening it. It's a REAL pain in that ass but just mask off the areas you want to paint and vice-versa for alternating colors. Pain in the ass up front, but well worth it if you can save having to eff with taking it apart.