Retro fitting an AC setup.

Not an AC expert, but shouldn't I be able to convert the R-12 to 134a. Since I have to make my own AC lines, I can control what fittings I use, VIA R-12 fitting on the end that connects to the unit in the dash and 134a fittings to connect to a new dryer/condensor and compressor. Or is this a huge no-no?
Sure. I just tossed that in with all those other short comings to tip the scale.