No reason why you can't. May be some reasons why you shouldn't. Aside from the fact that it's a lot easier to do it than to undo it, you'll want to be extremely careful with the grinder. The downtube is highly stressed and less than one millimeter thick. Even a tiny nick (the kind that an angle grinder can make before you even know it) can result in failure. If you have to do it, I would get close with a power tool and finish with a flat single-cut hand file.

FWIW, I personally would use barcons, as that is what the bike was set up for, they will look just right, and work very well. My second choice would be as others have suggested - just clamp on the shifters above or below the existing stops. That way, when you eventually come to your senses, switching to barcons will be simple.