don't forget to split the flange between the front/rear tubes from the center tubes and drill one hole on each set of flanges a couple of drill sizes over.
this allows the individual pipe[s] to move around a little, helping a bunch to keep them sealed to the head.
i stole this idea years ago from mr. P-body, and it has done wonders for sealing, using gaskets or just plain hi-temp RTV.
works really well on thin flanges to keep them sealed up.