rainbowbob,
Please, please don't widen the rear sight notch using "a friend with a Dremel".
I doesn't need to be opened up much - just enough to let in a little more light around the front sight, and the sides need to be widened equally or you'll screw your windage up.
Keeping the vertical sides of the rear notch square is also important to a good sight picture.
If it were me, I'd use good needle files with one dead side, and just go slowly, taking a few, equal number of strokes off each side.
KitchenTable Gunsmithing 101: Metal comes off easier than it goes back on.
KTG 102: Always modify the cheapest of two interacting parts (and here, we're talking about the frame!).
KTG 103: Fear, uncertainty and doubt is God's way of telling you to use a real gunsmith...
Good luck!