Made the repair. To get the nut to fit inside the hole in the hull, I drilled the existing brass nut with a 5/16" bit, then a 1/2" bit. That bit also took a little plastic with it. When drilling through the sides with a 5/16" bit, I held the bracket down and used it as a guide. Once it got about a half inch in, it hit brass, which I assume was part of the old nut. I did the same from the other side, then pushed through the brass to make it all the way through.
I slid my 3" 5/16"-18 stainless bolt through and installed the stop nut and the final result looks pretty secure. The metal bracket is barely above the hull.
Here are some pics of the install:
Drilled out brass nut in hull:
https://goo.gl/photos/UX5R5UJvMTz9fA48ABracket securely attached to the front post:
https://goo.gl/photos/qwL53XdMTYX8jFxg6Final result close-up:
https://goo.gl/photos/Sf8DKt8eqt6FLGF76Final result further away:
https://goo.gl/photos/aVpJaREsDr1HskK56I hope all this will help someone else who has this problem. I'll report back after testing it.
After it is all done, I was wondering if the pullout was caused by the ramp I use to put the boat in the water. I built a wooden ramp and slide the boat (with the mast on) up and down the ramp. The ramp is probably 20-30 degrees. Thoughts?
Also, I was wondering if I should seal around the bracket to prevent water entry around the new nut and hole it sits in.