Keep in mind that thermostats have absolutely NO effect on your systems ability to cool, it's simply a regulator of the temperature range the engine operates in.

I'm not saying that you should not replace the t-stat if you have an overheating problem. It's only 1 part of a whole list of things that need to be checked. 100* is fine every now and then. It's when the temps creep into the 105-110 range that you need to stop and cool down. If you do allow it to 120*, especially with a 601/2/3, just hope you have no permanent issues later on.