I experienced the exact same syptoms in my '49 GMC 1 Ton w/248, I changed the water pump, no change, I replaced the 4 blade fan with a 5 blade, no change, I had the radiator re-cored with a 4 row tube core, still warms up when it sits at an idle after normal driving. If I rev it up to about 1,000 RPM while stopped, it maintains a lower temperature. It doesn't overheat, and I have concluded that it is a result of reduced coolant flow through the engine at idle speed. I tried letting it idle when I got home one day, it warmed to 195 degrees as usuall when I stopped moving, and after about 10 mins was back down to 180 degrees and stayed there on a 110 degree afternoon, I'm not going to worry about it anymore.
