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First lets determine what the temperature of the coolant actually is.
Pick up an inexpensive dial meat thermometer and stick it directly into the coolant through the radiator neck.
Knowing the actual temperature rather than it's in the red is important.

By the number of posts that you have I am assuming (perhaps incorrectly) that you are new to the Model A engines.

Many new to the hobby tend to fill their radiators very close to the top and as the coolant warms up it expands and will push out through the overflow tube. This looks a lot like it is overheating when it is actually not.

You do not need to or want to "fill" your radiator. Just keep an eye on the coolant level and resist the urge to put more in as long as the top of the tubes in the radiator remain covered with coolant. The engine will push out excess coolant until it finds the level it is comfortable with.

As far as coolant it is hard to beat a 50/50 mixture of coolant and distilled water.

It is possible that you have a head gasket that is leaking. At temp and under load blow by can push water out too.

Have you checked the torque on the head nuts?