you can not put 2 stock 3 spd. cases together. You can put the front half of a mini t-case in front of the 3 spd. t-case via a marlin adapter or use the A/A rock box or marlins toybox wich is the previously mentioned option.
ill be doing the 203 cruiser case adapter from jerry nichols (who pioneered the aa adapter) in a couple years on my 60,...expensive, but on a daily driver that you dont want to gear down to nothing, i think it will beworth it.