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.

http://www.jasoncaps.com/php/product.stdopt.php?id=12

Go to Google "Images" and enter search criteria such as listed below to see various manufacuers products:

roof rack
truck roof rack
camper top roof rack

Here's a few pics: