Sorry for the dumb Q's
where do you think the "blow by" is coming from? lack of valve stem seals on the exhaust side on our 90's? Can the better scavenging effect of headers really do that? It can't be the rings, can it?
I remember in the old days....back in the 60's & the 70's
......when a new exhaust sys was installed they used to smoke a bit for a little while till they were 'run-in'...