I would simply source some R-12 refrigerant and keep the system original. Sure it costs a little more, but it should be a one time investment for your AC system, and we know it works perfectly. It is still available on Ebay and from other old car enthusiasts. Just lump it into the entire cost of the TT swap and it suddenly doesn't seem so bad.
Of course it would be a great idea to change every o-ring at this time to help preserve this precious refrigerant.