I can't remember if the flange on the cat on the eg is turned or not, I think that it is, But the problem you are going to have with putting a gsr header on is that it won't reach your cat (it's not long enough by almost 2 inches), and the GSR has the o2 sensor mounted on the cat, rather than on the manifold downpipe. your civic has the o2 sensor on the pipe, You could get a header for a b series non-vtec (either 94+ if your converter has a crooked flange, or a 92-93 header if yours is perfectly horizontal)
Or you could get a gsr catylitic converter.
either way you look at it you can't use the stock b18c1 header with an Eg cat because there isnt going to be a place for the o2 sensor.