340 overheating problem

The EVANS ran me $42.50 @ gallon and you DO NOT mix it with water. An added benefit it has is that it does not break down and need to be changed every few years. - Nor does it cause corrosion like conventional antifreeze/water mixtures. It's supposed to last the life of the vehicle without the need to ever change it. Because it resists boiling it is possible to remove the radiator cap no matter what the temperature reads without it acting like a geyser.

One thing that you may need to look into is having your block and radiator flushed. Conventional coolants tend to be corrosive and build sediment inside the cooling passages over time. Once that gets bad enough the water passages can no longer remove heat as efficiently.