The {late model 2 valve} V-10 makes max torque of 420# at 3,250 rpm and max HP of 305 at 4,250. You can run at 4K+ all day without harming that beast of an engine but you will burn a ton of fuel.
At max torque of 3,250 my fully loaded C, towing my motorcycle in the 10' cargo trailer (bike and trailer weigh 2,220} will climb almost any 6 percent grade at 55 without excessive noise and with reasonable mileage {been averaging 9ish for years}. Rare indeed is the 6 percenter that lasts more than 10 or 12 miles so you will be over the hump so to speak, in just a few minutes.
Tow haul is your friend and unless you're running very light, on the flat or a slight downhill you should utilize it for optimum performance as it resets the shift points. On steep downhills, it provides engine braking without burning a drop of fuel, saving your brakes. It also will keep your transmission 12 to 20 degrees cooler extending its life.
To get the best out of this fine engine just put your foot in it as needed and use your Tow Haul.
:C