13 Pounds is about average for what cars used to have in the 70s. A very different system in the Mangusta. Every pound of pressure raises the boiling point by 2 degrees. High pressure negates flow as it creates resistance, slowing the flow throughout the system
Much higher flow is available if you can use a lower pressure cap. A coolant recovery system will keep more coolant in the system, aid in cooling and also keep air out of the system that rots the hoses from the inside.
You could always show the car with the proper cap. Remember that cars like the Mangusta were not "engineered" in the classic sense. They were a collection of available parts that had very little testing as combined.
Dick Ruzzin