Yes MBAutowerks would definitely be your safe bet.
There are a number of reasons for NOT convert this car to 134. These are really bad candidates, not only do they have marginal cooling capacity which will really show up in Hotlanta, but also the evaporator has copper components. The oil and 134 combine to make an element that will corrode copper.
R12 would be a small percentage of your repair, just stay with it and you will be much happier.