So far, no I haven't had any problems. Without the boards, the overhang was just barely resting on the top of the cab when parked. I put some padding in there to get home, and as I was driving I could see the top of the interior cab moving a little bit. With the boards, there is about an inch of clearance and I've never seen any evidence that it has come down and touched the cab.

If the 2x6's hadn't been enough, I would probably have added a piece of 3/4 inch plywood under them to get a little more lift.

The problem with lift though is it raises the center of balance. It doesn't take much lift to introduce a lot more sway.