My truck had the engine removed at least 10 years ago in preparation for a swap which never happened when he had it. It seems that the M715's were put out of Military service pretty quickly and sent off to fire dept's and other government agencies who did what they saw fit to them when necessary.
It looks like what I have seen is that in the mid or later 70's these trucks went to auction to the public, but I am sure Jon would know the answer to that definitively. That leaves a lot of time for plenty of mods to have been done.
I'd wager a bet that some individual did the swap in yours either as a private owner, or possibly even for a fire department. There is a local fire company here that has a 715 brush truck, and a local mechanic keeps it going for them. He did a Dana 60/14 bolt swap, and small block chevy in it 10 or more years ago.
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