"For my Ford Lehman engines, Should I dilute the coolant 50/50 with distilled water?"
Distilled water YES! Pre mix NO, who knows what they use as OK ?
50/50 depends on weather you need the cold weather freezing resistance.
Antifreeze has less cooling ability so warm weather folks can choose 1/3 antifreeze , 2/3 distilled water.
Your water pump will still be lubricated and the corrosion goodies will do fine to protect the engine.
More BTU can be passed with the higher water content in the mix.
AS your antique has a very low pressure cap, usually under 5 lbs, there is nothing gained by using a high temp boiling point antifreeze as cars do.