Well, it depends on how adept you are. The very best way to achieve the coldest A/C is to build brackets and mount a 134A compressor, then replace your stock condenser with a 134A condenser of similar size. This coupled a 134A receiver/drier along with your stock R12 expansion valve and R12 evaporator will turn your A/C system in to a freezer. Building your own A/C lines can be tricky. Having a shop make you custom hoses is the best way to go. All in all, you are looking at about $400 to $500 investment. (using mostly used parts)
Similar to a custom A/C system I did in a Studebaker Avanti II, I got 18 deg F at the vents with the outside temp being around 90 deg F. Although having it this cold isn't the wisest, but you should be able to get at least 30+ deg drop in temp.