Best coolant for our 60-70's Mopars?

Well, a zero pressure would mean it is wide open.... so that would mean any expansion would create no pressure. I did not know that. Thanks for the education....now I need to read up on that aspect.

Edit to add: Here is from the Evans site:
Q. Do I need to change my radiator cap when using Evans coolant?
A. A different radiator/pressure cap is not required with EWC. A water-based coolant generally requires anywhere from a 7 to 15 psig pressure cap. Higher pressure raises the boiling point of water-based coolant. EWC expands slightly as it warms creating pressure of 3-5 psig, and the existing cap does not need to be changed.

So I am not sure where the zero pressure cap info is coming from or if that is for particular applications.