Best coolant for our 60-70's Mopars?

The price of Evans coolant is higher. The cost is lower. As for the price, I was going by the case price at the link ($44/gal). 65 ÷ 44 = ~1.47, about 50%.

How much to buy: Well, cooling system capacity is 13 to 16 quarts (with factory radiator) depending on engine and radiator sizes. Four quarts in a gallon, so buy four gallons and you'll either have precisely enough or you'll have enough plus enough left over for minor top-ups over the years.