A short bed camper will be different in two ways than long bed camper. As has already been posted, the floor is designed for less than 8' of truck bed support. However even more important, the camper was built with more weight biased to the front which allows you to keep center of gravity over or forward of the rear axle rather than behind it.
Removing the headache rack will be the best choice if you want carry a camper. You do not want to set the camper any father back or higher up than required. The rack will interfere unless you do just that.
Take a look at your rear axle and tire ratings. This is what will limit you on what weight can carry. Expect 2500-3000 lbs on the rear axle when the truck is empty, so that gives you an idea of how much weight you have left for a camper. I know some manufacturers made light duty 3/4 ton truck with semi-floating rear axle. If that GMC does not have 8 lugs on each wheel, it may be one of those. The LD versions are not suited for carrying heavy loads and you better off with a trailer or different truck.