Get a mid-late '70's J20 box if you can find one.
3.5" bore as opposed to the 3" wag/chero/J10 bore.
Bolts up the same and uses the same pitman arm.
Some discussion over the years about the input spline count/size but mine went in (70 frame, 85 steering column) just fine, input spline differences can be solved with the correct rag joint.
Here's the catch:
J20 boxes originally were all 4 tab boxes but over the years they have been substituting the wag/chero/J10 boxes, it took me 3 times for my McParts place to come up with the 4 tab 3.5" bore box.
Good news is they were also used on big GM cars like the Old's Toranado, Big Buick wagons and the Caddy commercial chassis hearse rigs, so they are out there.
Also:
From just some quick image surfing it looks like the M715 frame mount is the same as my '70 wag version.
Sort of inferior design, this is what mine looked like when I swapped in the J20 box.
I was able to fix that but check yours carefully.