I have always hear that one should drill and tap with 6-48, and if they ever strip out, to drill and tap with 8-40 in the same hole.
100 rifles I have drilled and tapped later, I have never had 6-48 fail.
But I have seen broken off screws and bolt in automotive situations, and trying to drill and and then tap the original thread never seems to work well.
I would drill for easy out, put in some Kroil and put on a little heat and see if it will unscrew out of the hole.
If not, I would drill out that screw and make it an 8-40 hole.