It might be necessary to make snug-fitting wooden plugs in the shape of the exhaust ports and use a die grinder to match the manifold to the head openings. That's a trick we used when porting heads and headers/exhaust manifolds for round track engines. Once the ports are matched up, them tweak the bolt holes to fit.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!