45 is just fine. 50 just uses more gas, but its still nowhere near red line. With the 7.50x17 tires, its even lower rpm than the above suggests. I belive that Randy's Ring & Pinon has a drive line calculator on their website. You have to remember, the truck was made to run with a full load, flat out. My GVW is only 6700 pounds, but I know that it hauled 110 bushels of wheat at a time, over gravel roads. That's about 6000 pounds of load I belive. As for speed and the splash oil, think about the guys that ran them at bonneville, and on the strip, at well over 6000 rpm.
When you hit 50mph in the truck, you'll know just how tight the king pins ,draglink, etc are.
My truck does cost more to drive at 50 than at 45.
