Do what you will with the R12A, but in my professional capacity, I must say that GM does not approve, due to its flamability.
If you do go to 134A, before you replace the condenser, I would look into installing some electric cooling fans. A good choice would be those complete sets available for police applications. They were designed for long idle times specifically to reduce underhood temps AND improve A/C performance while at investigation scenes for hours at a time.
The Impala PPV (9C1 option code) has a setup with a 320 watt (IIRC) fan set that is very good.
This suggestion probably cost no less than a bigger condenser, but may be easier to find, and cures several thermal issues at one time. It could be switched in a vareity of manners, depending on what you want.
"If the women don't find you handsome, they should at least find you handy." - Red Green