I owned/drove a 2000 F250 with the camper special for 14+ years (just traded it off 2 months ago). I owned 3 different 5th wheels in that time, but none of them were anywhere near the weight you're talking about, and I wouldn't have considered it safe towing/hauling that much.
I'm not a mechanic, but I believe you could increase the safe carrying capacity quite a bit with new rims and tires. I had heavier-duty tires on mine -- the biggest I could go with the original rims. I did that for a little extra margin of safety but still kept my weights under the factory specs. HOWEVER, the only thing I'd consider as an adequate upgrade for that size 5th wheel would be to convert your F250 to a dually. A neighbor of mine did that. He found a wrecked F350 dually and switched axels, springs, rear end, wheels, tires and the bed (w/fenders), so he basically converted it to an F350 dually.