I'm using a 180 in my 261. No problems at all moving home an entire cord of firewood in 95 degree weather yesterday. (Truck bed + 5x8 trailer) I run a stock size new radiator with an electric pusher fan in front of the radiator. No mechanical fan. My temperature gauge runs slap in the middle of the range, and only soaks up to 200 if you shut it off hot and leave it in the sun for a while.
If I remember right my shop manual says 160 for alcohol and 180 for modern antifreeze.
