Bad juju to modify a stock like that. If you want the easiest and best solution, simply remove the original "swivels," thread paracord through the holes, and tie your sling to the gun. Turns out much more useful than wrecking the original stock, and can be returned to normal with no problems.
Drilling holes in a "boat paddle" is costing you about $100 of value on the current market, if not more, just for the stock alone.
