This is actually a pretty interesting question -
The TMs do not give a GCWR for the M917. The TM does state the pintles on all the M915 series trucks are rated at 60,000 lbs capacity.
The various chassis types all have the same GVWR -
M915/M915A1 50,000 lbs
M916/M918 56,000 lbs
M917/M919/M920 75,000 lbs
The TM lists GCWRs for the M915, M916, and M920, but not the M917, M918, or M919. While I wouldn't presume anything, I think it does make sense that the GCWR of the various types of chassis would be the same.
M915 105,000 lbs
M916/M918 106,000 lbs
M917/M919/M920 130,000 lbs
Were that true, a fully loaded M917 - 75,000 lbs (with 40,000 lbs of payload) would have a towing capacity of 55,000 lbs.
All that being said, when you look at the M915, M916, and M920 GCWRs, (and military tractors in general) they seem to be based around the capacities of the trailers they're meant to pull. I suspect that in most cases, the absolute "break stuff" capacities are much, much, much higher. Likely with the M915 series trucks, even the "safe" limit is much higher than the listed GVWR/GCWR. So I suppose your answer depends on what your intended use is (on or off road) and/or how the law in your jurisdiction will interpret as far as dataplates, TM, or registration GVWR, etc...
Just my 1/50th of a dollar.