Was it tested for being straight/warped? Was the old manifold straight, if not, have it planed at a local machine shop. It both manifolds are straight, test the head for being flat where the manifold fits. I've seen warped manifolds and warped or eroded (a long-term exhaust leak will erode an exhaust port) heads many years ago.
2000 Winnebago Ultimate Freedom USQ40JD
, ISC 8.3 Cummins 350, Spartan MM Chassis. USA 1SG, retired;PPA,Good Sam Life member."We the people are the rightful masters of both the Congress and the Courts - not to overthrow the Constitution but to overthrow men who pervert theConstitution. "Abraham Lincoln"