Unless that truck is in unusually good condition or you have a special emotional attachment, I'd say the time, effort and cost would not be worth it. The simplest and least costly "fix" would be to get another 46re transmission from a salvage yard. If switching to a manual transmission is a "must", a better solution would be to just get a similar vintage truck that came with the manual transmission. An experienced and innovative mechanic with the time, tools and technology might accept the challenge and make it work. Since you're asking some pretty basic questions, I'm guessing you're not that guy! I could be wrong but as Dirty Harry once said "A man has got to know his limitations!"