In addition to the thumbnail pic, attached below is a link to full sized Catalina Drawing for the Strut Installation.

You'll have to remove the stuff above the strut and probably chip away at epoxy covering the bolts/nuts in the keel - depending on your year you most likely have to remove a water tank, etc. etc.

The picture in the link http://www.c36ia.com/node/1087 gives you and idea of what the cavity on the hull the Strut sits into -- you should be able to adjust this fore and aft in some slots in the hull to alight the angle to have the shaft running through it perfectly.

Here is something on Engine Alignment when the shaft is true in the middle of the Strut -- http://www.c36ia.com/node/1273

Here is a Maintenance Article, with lots of good pictures, showing what other owners have done: http://www.c36ia.com/node/1391