To convert a R12 system to r134 you need to replace the condenser with a larger one or one that has 20% more cooling surface area. You also need to replace the receiver dryer and flush out the evaporator and the rest of the system including the compressor. R134 requires the use of Pag oil not the mineral oil used with r12.
By the time you do all this with the factory parts you could have just replaced the whole system with a modern aftermarket system and been time and money ahead.