How much to cut out for fenderwell headers?

The headers are 1 7/8 tubes.Yes,braces will be in order for strength.

You will have to do some surgery around the shock towers. As far as braces it's been my experience that cutting around the shock towers creates stress in the unibody. Buddy of mine has stress cracks in the area of his shock towers. Of course his car wheel stands quite a bit and coming down that hard does put stress on that area of the body.