Simple question, how do you know it overheats? The old stock gauges are not very accurate. I replaced the thermostat housing with one with an additional port. The dash gauge is attached to this port and I installed a new T sensors connected to an Autometer gauge under the dash. Or use an IR gun to check the temperature.
About the coolant concentration, water is better than coolant. In the summer you can run more water than coolant (e.g. 60 water 40 coolant). Some products like Waterwetter claim to decrease the temperature. I haven't tried it, maybe somebody can comment.
1966 Convertible (Plant: San Jose, CA)
289 2V, PS, AC