Best way to cool it?

If you are mainly getting warm when going under 30 mph but fine when going faster, it is not your radiator that is the problem, it's air flow. Get a shroud. Also, does your fan have a clutch? If so, that could also be the culprit. If it has gone bad, it will not engage the fan properly at low speeds so you are not pulling enough air across the cooling fins of the radiator.