I think that combination will work for the OP. If he starts to travel the Rockies more often, I do think the 4.10's would be better suited for the gasser. I doubt he would lose any mileage doing this gear swap unless he drives fast while unloaded.
The payload thing between a gasser and diesel is bunk. The weight changes up front, but the rear axle weight remains about the same - I don't know anyone with a cabover heavy enough to worry about front axle weight.
Carrying the AF811SB on a long bed is no issue. If the OP already has a SuperTruss for his GM, it should be the same length needed to carry an AF992 on a long bed. The OP will need to buy new tiedowns and receiver, but the FastGuns can be used on the new truck.