Get an Allision. They are available but there will be the need to get one that has a stand alone computer. IE not integrated with the ECM Computer.
6 forward speeds and should be bulletproof. Of course you will have to re-gear the speedo, but that is the easy part. Just replace the OEM Speedo with an electronic one
It might also get you a few More MPG, and prolong the life of the powerplant. If you rebuild the T400 and add a gear vendors while you are at it you will spend more than what a used Allison should cost IMHO.