You need to start looking at the military surplus dealers for parts for the M-series trucks, the standard parts places like O'Reily's, Napa, and Autozone will not have anything for the vehicles except for some basic things.
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Doesn't the long CJ5 speedo cable fit the M715? It's been years since we've talked about it. Not sure if it was officially the CJ6 or an option for the CJ5 variant.
Found the NOS speedo cable at Saturn Surplus for $22.50:
http://www.saturnsurplus.com/m715/m715.htm
Also, yes, the long cj5 cable works as well...though I cant tell you the length off hand...there is a short and long and so far I find that one of them is 63 inches long...not sure which that is...
I went to Midwest Military. Also replaced the fuel gauge and sending unit after some confusion (caused by yours truly forgetting to mention that the beast had been converted to 12 volt...). Dropped the fuel tank and flushed some nasty stuff out, then put in a return line from the fuel filter to the tank, and moved the line away from the exhaust, to eliminate the vacuum lock caused by the 360 ci engine. The thing is loud as hell with new, dual exhaust parts, and top speed in 2H is about 55. Needless to say, with that engine and her gearing, the torque is... nice :D. Looking for a snow plow to put on her for the winter.