D Felix,
I believe you are incorrect. My Chevy 2500HD, single rear wheel, has a GVWR of 9200#. I was contemplating going with a dually because the GVWR bumps up to around 11,500#. If my numbers are correct, I believe they are, that's a whole nother ton. Problem is the thing is just so darn wide and IMO would be a pain in the butt to get in and out from the sides. I often use my wheel and sidebars to get in and out the box.
As for the original poster, you asked how many yards can fit into your F350's 8' bed, the answer is:
(1) Refer to your owner's manual or go to Ford's website. It should have a dimensions and capacity section for your truck. It usually provides you with the cubic capacity of your bed. My GMC with a short bed was just over 2 yards. Keep in mind this is level, so even more if it's heaping, good luck keeping it all in though. Tarping it can be a pain. Or, you can just break out the trusty tape measure, they never lie.
(2) Next, find the GVWR of your vehicle from the website/owner's manual. Subtract your weight and everything else you added to the truck after you received it. To be highly accurate go to a weight station (most legit places that sell stone have them). Now, subtract that from your GVWR. That is your capacity. Someone please correct me if I'm wrong.
(3) Now, find out how much a yard of topsoil weights. If 3 yards is under your capacity than you technically can safely and legally transport the material.
BTW, what does everyone use as a estimate for a cubic yard of screened, dry topsoil. I've always thought a 1/2 ton was a good estimate. Am I far off?