My truck has the same GVW, Jim, on 4 tires. I am running 235/85R16 M&S tires (got rid of the rotten old 17's, couldn't replace them with radials).
This pic ...
http://i283.photobucket.com/albums/kk293/hotshoe36/Our%20trucks/DSC00862.jpg... shows around 2 tons in the bed (GVW~8500#). A little tail-heavy, as you can see, this load (and others like it) allowed me to run the old 216 faster, actually, than when it was empty. 40+ mph felt like a fast cruise while unladen. But loaded like this, the engine felt happy running at higher speeds. I levelled out at 50 mph with this load, and it felt right for 50+ miles. That was with the old gear set, 5.14 ratio. Accelerating to speed was no kind of an issue.
I do this for a living, mind you,
http://i283.photobucket.com/albums/kk293/hotshoe36/Our%20trucks/37oretrainsummit.jpg and I know that a heavily-loaded truck needs a steady hand, along with more room to stop.The one time I had to brake hard - cut off by a moron in a hurry - I was using all the (top condition OEM) brakes, but they were enough. I'll haul 2 tons in that box any time I need to, without batting an eye.
Now, I have an '81 250 backed by a 4.10 differential ratio - still the same SM420 4-speed - but, having thought on it at some length, I've decided I also want a Gear Vendors overdrive
https://www.gearvendors.com/ behind it, for those long miles I'm planning as a featherweight camper-hauler when I retire.
