The most important thing to consider is: your vent holes are tubes in the front pylons. You do not want to seal these pylons where your hulls cannot vent.

I used an epoxy that is used to glue fiberglass handles in tools...axes, hammers, and such.

If I was doing it again:

1st I would bevel the casting so I could pour the glue in without making a big mess. Then I would place a layer of silicon in the bottom of the casting to keep the epoxy from running on the inside of the pylon. Then I would assemble my boat...leaving the plugs loose, so it can vent during the process. With the boat upside down, I would square it and let it sit overnight, so the silicon can harden. The next day, I would mix and pour in the epoxy resin. Let it dry, then I drilled a small hole from the rear of my front pylons through both the casting and pylon to enter the vent cavity. Be sure to test for proper ventalation before putting in your plugs and putting your boat in the water.

For what it is worth,
David