The best thing to do is post some pics of the chainplates particullarly inside the cabin where they come through. Some of the O'Day chainplates had brackets mounted to the bulkhead for extra support. I'm not sure how the 240 was set up. The only guy who would know is Rudy Nickerson of D&R Marine.
http://www.drmarine.com/
Rudy bought the molds from O'Day when the went out of business years ago. He worked for that company and bought all their parts.
Back in the mid 1990s he contracted a builder in Bristol to build an O'Day 240 to his specifications, and about six O'Day 192s which he called the Weekender.
I remember checking out the Weekender and I know that he made changes in the chainplates by adding special struts inside the cabin to reinforce them. I also remember going inside the 240 which he called the O'Day 25 back then but I didn't pay much attention to the chainplate setup.
If the chainplates on your boat are embedded into the fiberglass, you're most definately going to need to cut into the fiberglass inside the cabin where they enter so that you can find out what the conditions is in that surrounding area.
There's a new epoxy out right now which is put out by West Systems called Six 10.
http://www.westsystem.com/ss/new-six10-epoxy-adhesive/
One of my friends used this stuff on his O'Day 30 and he swears by. All you need is a regular caulking gun. No mixing involved and you can buy extra spouts for it.