Header leak

The pass. side header is fairly easy to remove,, pull it,, and use a staight edge to check the flange seal surface,,. You'll likely find a poor weld or dip,, some irregularity that should be obvious by the impression/lack of,, in the old gskt,, get the dips welded,, or file down any high spots as nec..

Try to fare out (file) the whole flange flat if poss.. If you do a good job,, a reg header gskt should be sufficient..

cheers

As TB mentioned,, a tube can also crack..