A few months ago I also started a post about the long shaft box removal process.  The long & short of it (pun intended) is that the box can removed from the top or bottom, without cutting the shaft, with the engine in place.  I removed mine from the bottom with the front of the car up on jack stands.  Get the jack stand as far to the drivers side that you can, or else it will be right where you need some room for box removal.  AMHIK.

Remove LF tire.

Under the car, I removed the z-bar clutch linkage, and dropped the center link off of the drivers side spindle.   I have a 289.  If you have an I6, I believe your job is easier.  I jacked up the drivers side of the engine and removed the drivers side motor mount from the block and the engine stand from the shock tower.  As my car has the 65 motor mounts, I had to loosen the pass side motor mount, and by raising the drivers side of the engine, I tilted the engine enough to get the box past the oil filter.  It is much easier if you pull the pitman arm off the box first (I didn't).  I think I also removed the crossmember.  I was able to work with stock exhaust manifolds in place.  If you have headers, you will have one more thing to remove.....

Inside the car I removed the wheel, dropped the column support bracket, unplugged the TS switch pigtail, and removed the column.  I also think I had to unplug another connection (back up lights????)

Having an assistant in the car to keep the greasy shaft off the seats when you are manipulating the box from underneath is a real plus......
 

I followed the directions in the owners manual.  The process was much the same as the others have stated.  I did not have to do anything to the engine, but I can't remember about the z-bar.  I was only removing the steering box for a rebuild, so I did not want to change any of the other steering components.  My recollection is that the most difficult thing I had to do was remove the pitman arm.  Decades of time had firmly attached it to the steering box.  I used a breaker bar, 4' extension and all my strength which was much more then than now!  I was surprised at how easy it was. 

But don't cut it off.

hmartin025 wrote:

Be careful about prying on pitman arm, only 3/8 adjustment bolt at top of box trying to take the pressure generated by pry bar. A puller is much better solution!!
Howard

You are correct!  I neglected to include a puller.  I got mine from Harbor Freight.  It is not the best quality, but once that pitman arm is off, you may never need to use it again.  I did, but only about a half dozen times while messing with the steering.  And it was easy.  Now I have R&P, so don't need it any more.