A F-350 will have a gvw of 12,000lbs not a payload of 12,000lbs your legal payload on a F-350 is gonna be 4500-5000lbs a F-450 is going to be 6000lbs. A F-450 is a tough truck I have put 4 ton on my F-Superduty which is a old version of the 450 I have hauled 9800lbs in my other truck . I don't run short boxes on my truck anything shorter than 12' long is useless for hauling long or bulky material. If you run a long box you have to be careful loading heavy materials like sand and gravel. If you want a true work truck go with a true cab and chassis truck the frames are heavier the suspension is heavier designed for flatdecks etc. The other thing you may consider is 4wheeldrive I know with my trucks 2wd isn't the greatest but I'am living with it I don't have the bucks to go buy a 450 4x4 for 45 grand. I have been thinking about going to a F-350 4x4 to have 4wheeldrive but the lack of carrying capacity worries me. A F-350 has no fudge factor your always pushing the limits you put 6000lbs into a 350 she's loaded. When you hauling aggregate products like sand and gravel its real easy to overload the truck. A F-350 your pretty much limited to 2 yards max of sand or gravel you can go .5 yard more but you will really notice a lack of braking power. A F-450 with the same sized box can carry 2-2.5 yards so much safer alot more comfortable to drive no big worry about trying to stop. When I haul sand and gravel I know the bucket on the 966 Cat loader is 3 yards heaping so 1/2 a bucket is roughly 1 1/4 yards a 3/4 full bucket is roughly 2 yards. When I get loaded with the 590 Case backhoe it has a 1 yard bucket so its easy to get a rough idea on how much material you are loading on. The material is sold by the ton but the customer wants to know how many yards it is approximatly. The heaviest material is 1" 1/2 drain rock there is no way you can put anymore than 2 yards of drain rock into a F-350 because it is 2900lbs per yard. You have to know the rough weights per yard what material weighs are or you will be overloading the truck.