It will cost a lot! Why do you want to change the ring and pinon? If you want more top end, the taller fifth gear may be a less expensive way to go without giving up acceleration associated with a 3.77 ring and pinon. Lloyd Butfoy is the guru at RBT, he can tell you all you want to know. I believe Wilkinson works on the boxes and other vendors probably do as well. I believe they all get their parts from Lloyd. Two years ago, I spent four grand at RBT to get a new, polished case, a new second gear synchro and the tall, .642, fifth gear. Please let us know what it costs to do the ring and pinon, I would recommend that you be sitting down when you get the quote
Art