I agree with everyone, route slots before assembling.
On my first machine, I assembled first, big mistake. It is harder to route, plus I messed up my slot cutter when I hit a screw.
Hint: when you paint your cab parts, put strips of thin cardboard in the t molding slots. It will keep paint from clogging up the slots. Be sure to remove the strips as soon as you finish painting, they may get stuck when the paint dries, if you dont.