I just wanted to throw in my 2 cents here in general for what has worked for me in the past...

About 15 years ago I did a total axle/drivetrain swap on my previous M715. Here's what axles worked for me:

1990 dodge 1 ton truck, srw. I believe they were Dana 60 axles front and rear.

I can not recall the wms measurement, but both axles fit between the m715 springs with no modfications other than new perches. I did not have to remove the rear drums, and I did a shackle flip in the rear with 2 inch lift blocks, then removed a couple leafs.

The front I did a spring over. I don't recall it being too difficult. The dodge Dana 60 came with king pins which allowed me to use an aftermarket raised steering arm. I think I even used the m715 tie rod and drag link mated to a wagoneer power steering box. The steering geometry was spot on perfect. I later added a track bar to help with death wobble I was getting running 37 inch tires. I also removed several leafs from the springs.

The front drive shaft was the biggest issue. It was a very steep angle and fairly short distance. I had a custom high angle one built by Tom Woods, which worked perfect. I also had a custom one built for the rear.

I ended up ordering custom 16.5 x10 (to fit used hummer tires) wheels from marsh. The back spacing was only like 2 inches or somthing like that- they were very offset to clear the front springs at full steer with 37 inch tires. I wonder if that's what contributed to the death wobble (which was virtually eliminated with the track bar- at the expense of some articulation)

All in all the conversion worked pretty well. I did some towing and moderate off roading with it, as well as daily driver.

As an added bonus I used the dodge 360, TF727 transmission (with overdrive) and mated NP205 transfer case from the same truck, which made for a very inexpensive, somewhat easy, and effective conversion.

My biggest gripe with the whole conversion was the TF727 transmission did not have a torque converter lock up, which led to pretty high transmission temps when running on the highway, in 4th gear, with 37 inch tires.