I think you are underestimating the M-37 and overestimating the
M-715. Having seen, driven and photographed both M-715s and M-37s on the
trail, I have to give the stock performance nod to the M-37. It's more
flexible and nimble than the five-quarter and has better trail dimensions.
To add weight to these views, I will add that I drove both of these rigs in
the service (Vietnam era) and have owned an M-37. My work puts me in almost
continual contact with 4x4 owners, parts manufacturers and vehicle builders
and that I have been an avid four-wheeler since 1975 (before if you include
the Army stuff).
I have seen several hard working M-37s with 318s and 360s and under
reasonable use they hold up well. The '715 has the edge in beef and can
take more horsepower, no doubt, but your inferance that the drivetrain
parts are interchangable with the civvy trucks doesn't hold water. They are
completely different setups. In my resource books, I was able to find
conventional Dana 70 offerings in the J-Series Jeep truck line but no front
model 60s.
If you talk to an aftermarket axle builder, like Dynatrac, Curry
or others, they have nothing very nice to say about the axles under the
'715. "10-spline junk" is what they call it. Granted, the Dodge is some
worse but my point is that the M-715, despite being a few years newer, is
no high tech marvel. The Dana 60 and 70axles under the '715 are old,
low-buck, government issue stuff that no serious, hi-power 4x4 builder
would consider.
IMHO, both the M-37 & M-715 can handle a 50% increase in power is
very safely. A 100% increase is possible with a light foot - especially if
an automatic is installed. Beyond that, you need to jack them both up and
roll new axles under them, or gut the housings and insert new internals
(easier with the '715)
I will agree with you that the best bet is to keep them stock or
near stock. One of our favorite old-time tricks for the M-37s was to
install 251 or 265cid Dodge or Chrysler (Desoto) flathead sixes. This gave
it a period flair but with usable increases in performance - especially if
you went to 4.89 gears. I installed a late '50s era 315cid poly-head V8
into an M-37 back in the '70s. About 165 net hp and great torque made this
a nice, tractable conversion that looked pretty original. I aslo assisted
installing a Polu-head 318 into an M-37 - another nice setup but I
preferred the big sixes.
Let the debates rage!
Jim Allen
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