Is that yellow payload sticker gospel according to law enforcement? I tow a fifth wheel with my F450 and this subject has been hotly debated on our forums. Folks who have been stopped and had their vehicle weighed have reported that what is done is they weigh your rear axle and compare that...
Even with forged internals how would the rest of the drivetrain handle that much power? Sounds like it would be preferable to buy a used vehicle and then work over the motor and drivetrain since you'd be doing that anyway. Save a few bucks that way.