The OP's wheelbase (158.5 stated in his first post) truck apparently only comes in dual rear wheels. The other things such as tires would affect it only if they were lighter capacity than stock tires.
The real way to figure load capacity it to go weigh the truck as it's ready for the load (fuel, driver, etc.) The subtract that number from the GVWR. You also need to pay attention to tire and axle ratings. I didn't see axle ratings on the Vehicle Information Kit, but the GVWR usually isn't as much as the front and rear axle ratings combined. That is, if you load both axles to their max rating, you will have exceeded GVWR.
