Well first, am I correct in assuming it is a 6000 series truck?
If so, its GVW should be 16,000 pounds, and provided the truck with bed and dump wieghs less than 7,000 it should carry 9,000 or a little more.
From a performance standpoint returning loaded up that grade you mention might be a long slow haul. Even with a 292 you don't have a whole lot of power to pull that grade, and doing so at speed will require getting the gearing just right.
I'm not the die-hard inline fan some of the others here are. I believe there is no substitute for cubic inches, running it up a 10% grade loaded to the max at even 45 mph is going to require more than a 292.
But given that grade you'll be dealing with, I think I'd swap later axles under both ends just to get the modern brakes. And gearing.
Search the posts by grigg. He has put some big modern axles under a 1-ton pickup, resulting in a pretty capable looking truck. He is currently looking to build a Diesel powered 2-ton.
I don't know what you've seen loaded on one of these trucks. My 6400 has a 12 foot bed, and even with extensions on the sides it would be hard to get 10,000 pounds of most crops on it. Maybe grain, but without a much larger bed and tall sides it seems difficult to imagine. Back when I drove truck out of the field, trucks in this class didn't often carry more than eight or nine thousand pounds.
