As we all know Thunderbirds have a bit of a known heating problem. One major goal is to prevent the coolant from boiling over and being lost. The radiator cap is an important part of this equation. As listed below the higher the pressure the higher the boiling point of plain water.
No cap = Boiling point 212 degrees F 10 # cap = Boiling point 239 degrees F 15 # cap = Boiling point 250 degrees F 20 # cap = Boiling point 259 degrees F
I certainly would not recommend a pressure cap in the 15 to 20 lb range unless you are absolutely positive that your radiator is in 100% excellent condition. But simply running a cap in the 10 lb range is reasonable and gives you a lot more protection. Proper mixtures of Anti-Freeze will also increase the boiling point. Unfortunately higher altitudes will lower the boiling point.