Thats a good point, i suppose you dont actually need to support it against the gearbox casing which i was thinking you would have to do, you just need to brace them together so the cant push apart. The reason i was thinking using the 6 speed is obviously you can retain the 5 speed functionality if you put a support carrier in but also the 6 speed box has bearings in the end casing as well supporting the far end of the 2 shafts for extra support. Problem you have is with removing a gear you then have to redesign the selector shaft aswell to accommodate for the moved gear. Not a big problem but its something else that would need addressing.