The transmission in your truck unless modified will be the SM 420 which was offered from 1948-67. There is a member here who completely rebuilt his with NOS parts and they are available. Novak rebuilds a lot of these transmissions and has upgrades available to make them bulletproof. Here is a step by step slide show of the rebuild process.
https://www.flickr.com/photos/advance-design_parts_co/sets/72157632620943718/