standard chevy p/s pump

my 50 Merc has an F-150 1977 year vintage and i am running a standard p/s pump, I am driving my Merc all the time It has a bit of wander, thats not the word. It "twitchy" is the word .I have tried Heidts adjustable steering equalizer box for pressure , Did not work . What if i put a larger P/S-...