Cooling issues 69 Dart GTS 340

'69 Dodge FSM says 190* thermostat and 16 psi cap. 14 -17 psi acceptable when testing.

Pardon my ignorance, but why/how does a radiator cap's pressure rating affect the cooling system? How can you tell what lb rating your radiator cap is?

Also, I have the MP viscous fan package that puts the fan's clutch approximately 1/4" - 3 /8" away from my new (last spring) radiator (Spectra).