I'm planning the same thing (although on an older Featherlight), and I'm going with the rail route... probably a little extra cost in the get-go, but I think it's the more versatile plan in the long run: You can swap the GRS to any other rail-equipped gun, or swap a different rail-mounted sight to the Ithaca, as you please. I don't know how much a rail-mounted GRS costs vs a standard bolt-on one, but drilling and tapping won't cost you any more, and a rail can probably be had for <$30.