Steve, first of all, I don't have the answers to your questions but here is something that you might want to consider. IF you have to replace the coils, drier, compressor and hoses, you might want to take a look at how much that is going to cost you for parts and labor. As for freon, if you are running R-12 and want to upgrade to R-134a, you are probably going to have to replace some or all of that equipment anyway. One of our more techinically inclined members who have been down that road can tell you that.
IF you are going to pretty much have to replace everything to upgrade to a more modern system and R-134a, you might want to take a look at replacing the whole system, as some on here have done, and are very pleased with, using the Classic Auto Air "Perfect Fit" aftermarket AC system. They make their units for the 1955-1966 Tbirds, and the cost is about $1,400. For that price, you get a brand new, modern system that will run you out of the car, running R-134a instead of the more expensive and harder to find R-12. So once you have found out what all you are going to have to replace just to improve what you have and continue running R-12, or upgrading that system to R-134a, compare that to CAA's "Perfect Fit" system cost. Then you can make a decision as to which way to go. That is how I see it. Personally, if I were going to have to replace most or all of the AC equipment on my car with upgraded components, I would do so using R-134a, and go the expense of putting on the "Perfect Fit" modern system, and be done with it. Instead of relying on a 50+ year old AC system... Here is a link to the CAA.