The freeze 12 is the auto repair guy quick fix for the old a/c system he will never see again. It will work, and cool like R-12 but will be harder on the commpessor. The freeze 12 does not move the oil around like it should. Also if it leaks it can leak in a strange way, part of the component is small and will leak a small hole leaving the larger stuff behind. So if it leaks you have to pump down the system and recharge.
The 134a retro kit is only the 2 fittings, new oil/dye, 134a refrigerant and a sticker that says this vehicle has 134a in it. Although it will not cool as good as old R-12, with 134a you can just top off anytime. It was a tough call for me as well, but my friend has done this a long time and he has been a good teacher, so I done it the way he said to.
Dave