I have 1999 F-250 V10 Super Duty. This truck is my occasional driver (once a month). For awhile I've wanted to order a safari roof rack that fits over the camper shell (topper in the eastern states), is there anyone here with a set up like the one I want here?. I'm in the Mid West until April, and have the option of stopping by Arizona or driving up to Northern California for a custom set up. I only want it mainly for looks and for sleeping on top of it, maybe twice a year during the Mojave or Death Valley Desert trips.
I've never done both, but I've ordered toppers with rails and I've installed rails on toppers. Perrycraft makes a couple of nice options for rails that will bolt to the topper. Then clamp the rack to the cross bar and you're set. Although, the racks I've seen wouldn't be long enough for me to lay in.
I think your biggest problem will be structural support of the topper.
If two of you are thinking about sleeping in the safari rack, the proper rack will do the job. But most toppers are not designed to hold that much weight.
You may want to look at some of the rack systems that mount to the bed rails under your topper, with supports going outside the topper to the rack above. Some of these systems are quick release/removal so you only have them mounted when they are going to be used.
I want to do the same mod down the line. Do you already have the cap (what we call it in the Southeast)? Some manufacturers like Jason offer a "Honey Comb - Open Tri Cell - Interior Support System" for the installation of roof racks. A "commercial" grade cap will support a roof rack as well.