On the left is a unmodified NAPA brand quad shock, and a 1/2" female spherical rod-end.

In the middle is the same shock with the top cut off and the shaft threaded with a 1/2" fine thread die.  Enough length of shaft is cut off so that the rod-end hits the shock body when fully compressed.  Your quad shock will need to have a 1/2" diameter shaft.  The rod-end is also cut down.

On the right is a modified quad shock with rod-end attached.  The new compressed length is 10 3/4".  A jam nut is used instead of a regular nut.  Because the jam nut is thinner, the compressed length of the shock is shorter.

Tools Needed:
1/2" Fine Thread Die
Cutting Oil
Hack Saw (18T works good)
Metal File or Bench Grinder

The file or bench grinder is used to flatten out the rod-end after cutting it with a hack saw, unless of course you are good at cutting straight.  The cutting oil is used when threading the shaft.  The shaft cuts easily with a hacksaw.