I would look into not filling the pylons wiht epoxy to support an aluminum tube as a way to stiffen the boat. If you decide to continue on teh route of making the holes in teh pylon round again, may I suggest an alternative method.

1)Attach (with an appropriate adhesive) an aluminum plate on the inside of the pylons that will cover each hole. The plate should match the curvature of the inside of the pylon wall.

2)Reinsert the pylon into the casting. Using hte hole inteh casting a a guide, drill a hole through the newly affixed plate on one side. Move to the other side of the casting and repeat.

3)To help prevent slop in the future, which will prevent the holes from elongating again, put a shim in between teh pylons and the castings. This will take out the slop, but you still can disassemble for storage.

Even if you don't fix the elongated holes, shimming will make teh pylon/casting joint tighter.