a rack requires quite abit more pressure, without the pressure the belt will chatter at low speeds and can even stop the pump
mopar mucle had an article for lowering the pressure by installing (thicker?) shims in the pump. my guess is you could install thinner shims for higher pressures
also you can get PS pumps built for racks now off ebay for mopar
it looks like your using an old federal pump, which is lower pressure from the newer signaw pump built in the late 70's and 80's