Hello cubed one,
There was an auto box produced for the gmc engine, it was used in one of the american amoured cars, sorry i can't tell you which one as american truck arn't my thing :blush: (possibly the staghound?). I came across it many years ago when george saunders turned up some gmc engines and these turned out to be from these armoured cars, from what i can remember the armoured cars had two gmc engines which were handed left and right and the auto box's were part of this set up. A member of the north west club rebuilt one of the engine for his gmc and told me about them being left and right etc.
Sorry i can't be of more help but it was all about 20 years ago, but maybe this might jog the minds of other forum members.
Edit: Just looked on wikipedia and it is the staghound, so the bedford research hasn't replaced everything else just yet!!! Eddy 8men has had contact with george recently and will be able to give you his contact details maybe he still has the auto boxs or at least tell you where they went.