Doc,
Consider $65 a lb. for R-12 vrs. $12 lb for R-134a. You need to find and repair your leak no matter which refrigerant you decide on.
If you decide to switch to R-134a heres a list to consider;
R-12 compressor - drain out R-12 oil DO NOT attempt to clean any of it out, just let it drain.
Replace the filter dryer line, I would also replace the oriface line,
Have condenser pressure tested for leaks by a radiator shop,
get flushing kit at a parts store and clean out the condenser core and the evaporator core,
replace all "O" rings and springs at connections, you can get a kit for this at most auto parts stores. Note that the "O" rings are made of a different material to be compatible with R-134a, the kit should have 8 ounces of R-134a oil for the conversion (use only the R-134a oil THAT IS INTENDED FOR CONVERSIONS) Put 5 in the compressor and divide the rest between the condenser and the filter dryer,
After the system is assembled vacuum down to 27 to 30 inches of vacuum for 30 to 45 minutes, then check for any leak down for 30 minutes, if none, vacuum for another 30 minutes then charge with about 28 to 30 ounces of R134a,
When you're fixen' thumbs_u (get ready )to do this Doc, let us know if you need any more info. rangerj