You wont be able to mount the 420 with all the 12 fans screws, screwed to the case, there's not enough space. You can screw 10, and the other top 2 just screw the fans directly to the rad through the opening, or you can use the fans on the inside.
I have a 360 and it maxed out the sliding holes as you can see:
But for both option be aware that if you go with the 420 it must be a slim one because it can interfere with rads mounted on the top of the case.
