make sure to cut the flanges between each exhaust port, and drill one hole on each tube section two sizes larger than they are at present.
this will make sure the pipes stay sealed, as it allows slight movement between heat up and cool down.
i stole this idea from member Mr. P Body at least 15-20 years ago, and it just flat works, no matter what gasket or sealing goo a guy wants to use.
this even works on the cheesy 1/4" flange tubes.
of course you want to make sure the raised bead or flange itself is relatively flat. a light dressing with a belt sander or da sander will work for that.