1968 Dart cooling system & gauge Questions

Each lb of cap pressure raises the boiling point 3 degrees. So 9* difference between the 13 & 16 lb caps. If the engine runs cool, probably be ok with the 13 lb cap. Agree with less pressure.
My car isnt overheating. In fact,as I mentioned earlier,the top hose never seems to be under pressure at all and I can remove the cap even when its at operating temp,which is unusual. Like Dana said there must be a small leak somewhere. Dont see any fluid leaks though