Just so you know, moot now,
I have drilled and tapped a few 904 convertors, with no noticeable balance effects; but then, the stock 318LAs only rev to barely 5000 when the valves begin to float. Mine had better springs and went to 5500 but the cam was done by 4500 or so. I just reved it to 5500 cuz she went there lol.