my grandpa has been the coordinator of his counties civil defence, now call EMA (emergency management agency) since the early 60's and had 5 of the 6X6's. he had them from the 1972ish to the early 90's the only reason he got rid of them was the price of parts. they were all 1953 GMC auto trans, I think either m211 or m135??? anyway there are LOTS of parts out there and the prices arent that bad if you only have one truck. look up the MVPA magazine its a great source. they are great trucks so go for it. shoot me a PM and I can give you some more info.
