91 Cavalier VL A/C Retrofit

I have a 91 Cavalier VL which I desire to retrofit from the existing R12 to R134a. I purchased the Quest Recharge and Retrofit kit and after installing the R-134a High-Side retrofit valve, I noticed that the R-134a Low-Side retrofit valve would not screw onto the existing R-12 fitting. Anyone know the specifics of the existing R-12 fittings used in factory installed A/C for the 91 cavalier is?

What is odd is that one of the High-Side R134a retrofit valves would fit onto the existing R-12 Low-Side port, but is way to big to attach my refill hose onto. I made quite sure that the Low-Side port is really the Low-Side port, as it is clearly labeled thusly and it is between the evaporator and the compressor.

I really would like to complete this retrofitting.