I had to replace the pads under the mainsheet blocks. But with the headliner I couldn't get to the nuts. So I chucked a fine blade in my jigsaw, put a paperback under it to limit the depth, and sawed holes for 4 inch access plates (the flange on the base is shorter for the 4 inch plates than the 3 inch versions). They fit tightly anyway, but I "glued" them in with some 3M. With the holes, I could then get to the nuts--which didn't exist. Your mainsheet blocks may just be screwed into plywood pads under the cabintop !!!!! So out came the short bolts and in went longer bolts, WASHERS and nuts. PS--You may also saw thru electrical wires to you nav table and galley lights! I did. Whoops.............