lol, the 172 has a jc5 type housing, the 6 speed boxes dont, they are the larger housing. (Cant remember the designation at the mo. doh !)
YES it will fit a 172 if you change the engine block for the f4r T (I have 3 brand new ones if you want one) - these are made for the six speed. you may well have to alter the drive shafts, but that really is straight forward (Cost about 100 squid for the work on the shafts)
Phil, sorry m8, not as far as I can ascertain.. you are stuck with it..
it is possible, on any unit, to make an adapter plate if you have enough length on the input shaft to cope, but this is fairly complex to get right.