I have a tub, two sinks, a toilet, on the 2nd floor. They are the only
fixtures that drain into a Cast Iron Stack which is now clogged. This
drain line leads to a shallow 30" crawlspace with a dirt floor and very
difficult access (must crawl 25 feet to get to it). The drain runs
vertically along the short foundation wall, then heads horizonally
through the wall, just below the ground. Basically, the only piping
accessible from the crawlspace is a cast-iron 90.

The problem: There is no cleanout to get a snake into.

I would like to add a cast-iron cleanout just outside the foundation
wall, since the drain line is only a few inches deep right there. I
will snap the drain line, and add a cast Iron cleanout with the propper
no hub connectors.

Does this sound like a reasonable course of action? Is there another
way I should go? I have only worked with Cast Iron a few times before.
I figure two snap-cuts, two no-hub connectors, and I have a cleanout.