The F450 also has ten lugs holding the wheels on, heavier springs, and much bigger brakes. I wouldn't go too much over what the door sticker says even with additional springs or air bags. Going over the max the truck is rated for becomes a legal issue, and here in New York they issue HUGE fines for being over-weight. Also if you were ever involved in an accident and it's determined you were over-loaded you open yourself up to lawsuits, not to mention the fact that your insurance company may not cover you due to negligence on your part by overloading.