Bypassing the heater core is only a small part of getting cold air from your A/C. The A/C system in the car was designed for R12, and just putting in R134A will not make the system work. Just putting a can or two of 134A will not make the system blow cold. You will need to replace the receiver/drier (134A will attack the bag holding the desiccant in the drier), flush out the evaporator, condenser, & hoses with A/C solvent (sold at most auto parts stores in an aerosol can), and replace the compressor if it is bad. You will also need to install the proper 134A service fittings and retrofit label. After all that is done, it would be best to take the car to an A/C shop to have it leak tested, evacuated, and recharged with the proper oil and amount of refrigerant. If not done properly, you will be spending a lot of money needlessly, and have a system that will not work properly. Even if you have a source for R12, it is best to make sure the entire system is working correctly (compressor working, no leaks etc) before charging it.