I wasn't sure about the height on my Alaskan/'88 F-250 and I wasn't about to drive 80 miles each way only to find out it hit the cab or the side rails so I did what any cautious guy would do...
1) scrounged some 2x6s and bought one sheet of 5/8" plywood.
2) I cut a frame to sit UNDER the plywood with a 2x6 running fore and aft at the edge of the plywood so as to carry the weight of the camper up through the sides of the lower section and not eventually push the floor out of whack!
3) I added TWO sections of 2x6, one at the front and one at the rear of the plywood to do the same thing; carry the weight of the Alaskan up through the sides of he lower, not through the floor.
4) I also added TWO more crossmembers spaced evenly under the plywood
As it is, I found I can remove the 2x6s and just put the plywood down onto my Bed-Liner. That way water will run down the liner to the rear and out.
The point is....carry MORE than what you need to be sure as you can modify it later....
Oh...and if you post a REAR pic of the Alaskan, I think Ken can comment on your having a rear width problem! However, if the rear of the camper is the same 48" wide as the rest of the camper you are good to go.