Why not warm the truck up, and stick a meat thermometer in the top of the rad, where the top hose dumps in?
The temp gauge on my truck is graduated, but not accurate. What it thinks is 180, is really 160. My Pontiac is worse, what it thinks is 200, is really 160. I've driven that car over mountain passes at 100 degrees outside, and it doesn't overheat same with the truck. The gauge just shows relative temperatures, not accurate ones.
As long as the coolant coming out of the top hose and entering the radiator is the same temperature as your thermostat, there isn't much to worry about.
I'd measure the "real" temperature before worrying.
