Fred, I'm not sure on the max load. I would guess around 2 tons. GVW is 9,900 lbs, but I haven't run the truck across the scales yet. I already tested it with a 2500 lb pallet of bagged coal and it laughed at that.
As for the rust, I have used POR-15 in the past with great results. Your WD40 trick works well also, especially if you spray it in the wheep holes before the rust gets started. A friend of mine sprays a mixture of diesel fuel and engine oil the frame/chassis of his truck every fall, doesn't smell too good...but it has been through many winters with very little corrosion.
As for the Ford v. Chevy thing...we have had both for farm trucks, and they each have pros/cons. Our current dually is a 2005 Chevy with the Duramax/Allison combo, it is a pleasure to drive and has been trouble free. It has also towed some loads heavy enough to discolor the paint on the rear differential cover