Slip it in through the mast gate....
...after removing the boom. Lock the padeye into place by tapping in a stop screw above it. On my 77 the boom's gooseneck is on a slider to accomodate the downhaul. When I converted to a cunningham I tapped a couple of stop screws into the channel to immobilize the boom. In case you don't know where the mast gate is, look on the right side of the channel 6-12 inches above the boom. There should be a small plate covering a gap in the track. Remove the screws carefully, because they are an odd size that you can't find at home depot. This is called the mast gate and it allows access to the track.Since you're a new owner you should have this link to our Intl Cat 27 organization. There you will find downloadable copies of owner's manuals, parts lists, tech tips, boat specifications and much more.http://www.catalina27.org