It is typical of a flattie to run hotter at speed because of the exhaust port arrangement. Having an A/C system definitely stresses the system further. I notice mine will run 180 deg at 65 on two lane hwy on a 90 deg day. It will run 205 at the same speed on the same day on the interstate. The difference has to be the temperature of the air 12inches above the asphalt on that 5 lane super slab. Those conditions did not exist in 1951 and you could not go 70 mph for hours at a time.
It is also significant to note I am running a 5 speed with OD that drastically lowers my rpm,s