If you don't mind sending it to Missouri, I would highly recommend Wayne Euper. I had Wayne fix my 63 muncie just recently and I am VERY happy with the results. He tore it all down in front of me and showed me the internals. I had to have two bushings put in the case as the holes for the counter shaft were stretched out. Mine leaked TERRIBLY!! He also did some other work. Reasonable prices and it was done in a timely manner. He's been building GM 4-speeds since 1962. His phone number is 816-776-6754. He's in Richmond, MO, just north west of KC.
Ross