A higher CCA 6V battery won't help. The reason the 12V battery works is because of the higher voltage overcoming a high resistance connection somewhere in the circuit. With the 6V battery, you're not getting 6V at the starter, but with a 12V battery it sounds like at least 6V is getting to the starter. The high resistance connection could be anywhere in the circuit, including the ground between the engine and battery.
BTW, you can edit your signature to include your truck info so we know what you're working on. If your truck is a 62, it should have a 12V battery.
