If you're not keeping the 2 rail coupler, I'd recommend you cut off the loop that holds the box lid down. While you can lever it off, it takes quite a bit of force and on the fragle hopper end platforms, you risk doing some damage. Once you cut it loose, the box lid should just lift off, and you can take out the coupler and unscrew the box from the end platform. Unless its really cheap, or something absurdly rare, its hardly economic to spend $25 or $30 on another pair of trucks to convert the car.