'04 F150 super cab 4x4 GVWR is 7200lbs, listed curb weight is 5700lbs. You have a 38**lb limit for the rear axle. I don't know how much weight is on the rear axle to start with but that gives you a few numbers to play with if your worried about legal aspects. I have pulled half way across the country with this truck right at or over GCWR pulling a tag car trailer. I never felt for a minute that I was pushing the limits of stability with the truck loaded like that. I also have a camper that weighs in at 1800lbs I haul. I have thought of going the same route with the sled bed, and would not hesitate to do it, but I am not worried about DOT they are pretty slack here. I am expecting that when loaded with two sleds the truck will squat to the point of being level (factory rake gone). The only reason I could see bags being needed is the fact that you are making the truck EXTREMELY top heavy and stability could be an issue, found this to be true with the camper. Keep in mind though any truck with 1000lbs of fun over the bed rails is going to be top heavy. Case in point I share ownership of camper with my father who installed bags on his F350 7.3PS Crew Cab dually because he felt it was a bit unstable with the camper. It's just going to be felt more with the softer suspension of the 1/2ton, thats why stiffening things up with the bags would help, I did.
Sorry for getting long winded but I thought I would share a perspective from someone who has many many miles on the truck and has used it in most any loaded/unload situation you can put these things through without killing yourself.