If you want to strictly be within GVWR with the heaviest TC, you'll need a chassis. That is not a tradeoff I'd be willing to do with a flatbed although I'd consider a pickup bed conversion on a chassis truck. On potential issue is the derating of the engine and the higher gear ratios (read worse mpg).
Otherwise, F450 with 19.5s is the only reasonable choice for me to allow the ability to tow, turn tightly, have stronger tires, and brake the best. (Also worse mpg with 4.30 gearing unless you have a heavy load or drive slow).
