AC Low side

Hi It's that time of year & I have 1 can of R-12 left & want to top off my AC on my 87 240 DL, but don't know which one is the low side. Does anyone know or maybe have the Haynes AC manual to refer to ? Also,, I've heard of people doing the changeover to R-134 with a kit from O'Reilleys or AutoZone. Are these worth the time & effort ? Then the main question with this is,, does it cool as good as R-12 ?? Getting warm here in OKC Thanks Rich