The cooling system on a 70 is not pressurized. The little tube that comes up through the radiator is the overflow tube. If the coolant level gets above that, then the excess runs out to the ground. You'll want to fill it up to where the coolant covers the top of the radiator core, but not so high that it runs out of the overflow when it expands. When everything is working right, the water doesn't come out of the overflow.

Since the tractor has sat for 14 years and the water came out of the filler cap, my guess would be the radiator is plugged up to the point where the coolant can't flow down through the radiator as fast as the water pump is sending it to the top. It could also be a blown head gasket forcing compression into the cooling system, creating surges in the flow. I'd start with flushing that radiator out really good, making sure that it isn't plugged up. Also, look for bubbles in the coolant at the top of the radiator when the engine is running, meaning some kind of compression leak into the coolant.