I have used a surfactant called "Water Wetter" on my rig. It is supposed to improve heat transfer from the metal to the coolant and vise versa. It helped some, maybe 10deg.
Also, I have run across situations where the crank pulley was slipping on the rubber it was supposed to be bonded to. So the water pump/fan did not turn as fast as expected. This problem took me much too long to figure out.
I have run Chevrolet 235s for many years and 200+ water doesn't seem to bother them. Running continuously above 2800RPM does heat the oil to 250 and up. I don't think that's good. Do you have an oil temperature gauge? On a 261 with an external oil filter you could install an oil cooler if you find high oil temperature.
