First thing that I'd suggest is doing a search. Use R12 and conversion as keywords, and you'll have a wealth of informed opinions to help you. The bottom line I think, is that R12 is what your system originally was designed for, and will work better in your system than R134. The newer R134 systems are designed with larger condensers that compensate for R134's less efficient cooling ability. R12 is still readily available to your shop, and although R134 is cheaper, enough R12 to fill your system can be had for less than $100 - less than the cost of a proper conversion.
Having said all that, if you want to convert, it's certainly a good time when a major component is being replaced.
82 300 SD
77 450 SL (gone)