It's a very odd combination of parts. The inner axle is about the size of a common Dana 44 of today, though bigger than the Dana 44 of the era in which the axle was built (most D44s of that era were 19 spline). The outer shaft is the same as was used in the Dana 70 axles of that era. The U-joint measures out to be a beefy 1480 series u-joint, most likely a 5-88x Spicer. This is a common joint used in closed knuckle Dana 70s to the mid 1970s and similar to the 5-332x size used in the open knuckle Dana 60 fronts used from
the late '70s until today.
Internally, the M-715 axle is similar to the Dana 60s use the the front axles of some high GVW Ford F-250s and F-350s in the '74-76 Ford, though the Fords had much smaller u-joints and smaller outer shafts. In '77, the Dana 60 fronts went to the more familiar and much more beefy 1.5-inch, 35-spline inner axle and 1.31-inch, 30-spline outer shaft. The Ford units mentioned earlier are known as the "Wimpy-Sixties" because of their small axle shaft dimensions and small axle u-joints.
The way I see it, the inner axle is the main weakness of the M-715 D60, and I presume that's where they break most often. It might be possible to substitute a more modern 35-spline Dana 60 inner shaft, perhaps a blank axle cut to the right dimension and resplined. You'd have to then install a 35-spline carrier (perhaps a Powr-Lok) or at least 35-spline side gears. Without all the parts laying here to compare, it's hard to say just what could be done, but it's likely there are a number of other "mix-n-match" improvements that could be made internally without changing the exterior look.
Super Swamper Bogger
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