Why are you changing the receiver drier?
Since you are in a Southern climate, I don't think you would be happy with 134 unless you drive strictly on the highway. Additionally, to properly change to 134 is a big job and if not done correctly will ultimately result in serious problems. Even if done correctly it can lead to problems.
R12 is now down in the $30 per pound average price range. Just make sure you repair any leaks and stick with R12.
When flushing the system or any other repair work involving opening the system, get everything done and back together, then put the filter drier in place as the last step so that it is exposed to the air for the least amount of time.
There must be a reason you are flushing the system. Please don't tell me this is a preventive maintenance project.