Correct PSI Radiator Cap for '70 340?

That 15 LB spec is also for stock, copper brass radiators. If you have a more efficient aluminum radiator, then by all means try something lower like a 13 LB. That's what I run on Vixen and it runs cool as a cucumber. We took it into town Monday in 90 plus degree weather and it barely moved the gauge to 1/8 up. The cooling system just works.