If your evaporator is the original one, inspect it closely before you charge it. Get one of the cheap borescopes and run it up under the evaporator from the passenger footwell after you remove the (easy to do) motor resistors. Look for any crud or rust. Mine was full of dirt and just nasty looking.
No doubt it was part of the problem for my pitiful AC before the upgrade.
If you KNOW your evaporator is clean and not looking like mine did, charge it up.
R-12 is a slightly more efficient gas, but it is expensive. Virtually no professional shop will have the equipment to service it. They all threw out the R-12 service equipment long ago. Look on EBAY and you might find the recovery equipment for R-12 so you can service the system in the future and not just dump it in the atmosphere.
You will need to do a full and long flush of the entire system. Cross contamination of the gasses makes for bad results. Most real pros of the era had a separate set of gauges to use with the different gasses to avoid any contamination.