Sparky,
Did you ever consider a B and J four speed trans? They are really not that expensive.
If you want I can run a simulation for you with each transmission to see what works best. Also, a 4L80 e might be a good choice.
The over drive shift is more discrete than the 2-3 shift that you have with a stock TH400.
Btw, sorry for the log absence from replying to your thread. Tapatalk was giving me major issue over the last year and locked me out of your thread.