Half way up
You will need to attach a lifting "loop" around the mast at a point half way up the mast (to keep it balanced). Above half way will make it bottom heavy, below half way will make it top heavy. If it is a dual spreader rig, someone will have to climb above the first set of spreaders to attach this loop. A good sturdy line going down to a secure attachment point (winch?, strong gooseneck?) Then you need to lift the mast about 1 foot off the mast step (unless it's keel stepped). Is there a BRIDGE, covered well, or something in the area you can nudge your boat up to and secure it? It would be risky, but an option (that I wouldn't take).