With a 8'-6" bed, the RAM 4500 you are looking at is almost guaranteed to have a 60" CA (cab to axle). So, the flat bed will extend 42" behind the axle centerline plus maybe 8" for a bumper, so total of 50 inches. And it won't be wider than probably 82 inches with stake pockets or rub strips. That makes the diagonal just under 65 inches to the corner of the flat bed. My Cruiser is 83 inches from the center of the king pin to the lower front wall and storage door. That is with the standard extended pin box. If yours is the same, you should not have a problem with the flat bed corners.
When my Cruiser is level, king pin height is 48 inches. Most likely, the RAM 4500 flat bed will be at least 40 inches high. So that means the GN adaptor needs to have an adjustment range around 6 inches to 10 inches, actually lower since the adaptor doesn't drop flush with the floor. However, if like many flat beds, the ball is under a trap door, then you can probably add 5 inches to the numbers.
