I have someone who has been doing transmissions for about 15+ years on just about anything. Let me know if you are interested. He recently rebuilt my 3 spd and it's perfect. He is also really close to you.
inglewood transmission in fullerton does good work, they built bullet proof diesel transmissions for drag and puller trucks. not sure if they do smaller transmissions but might be worth giving them a call, and they're local to you.