You may know all of this already but I thought I'd throw it out there in case you don't. Cooling system pressure came about because vehicles would overheat when changing altitudes. Water boils at 212f at sea level but at 3000 ft it will boil at around 206f. It all has to do with atmospheric pressure being higher at sea level than at any given altitude.
For each pound of pressure that you can place on a cooling system, you will raise the boiling point ABOVE that of sea level by about 2.5 degrees Fahrenheit. So, a 10 pound pressure cap that works properly will allow you to heat the coolant up to approximately 237 degrees before it will boil.
Armed with that knowledge, you can see that you don't need much in the way of pressure to protect your system from boil over. AND, the less stress that you place on the system by using the lowest pressure that you can get away with, the longer it will last and have fewer leaks.
After all that, I have to say I don't have the spec for your 53t engine but I know it will not be over 10 psi. Personally, I'd go with an 8lb if I could find one. That gives you about 20 degrees above boiling at around 232 degrees. The overheat alarm SHOULD sound, for that engine, at around 215 to 217 degrees and, if you are running an EPM (engine protection module) as in a SENTINEL or a MURPHY switch, it should depower and/or shut down at around 230 degrees.
I would have no qualms using a 10lb on that engine since I know most of the old 71's I worked on used that pressure for the surge tank.
I hope this is something that helps you make a choice.
