You could do it the way it's been done for decades- - - - -bolt the manifolds together and mill the manifold/head mating surfaces for a correct fit. At the machine shop where I ran the connecting rod reconditioning department, we had a big belt sander (16" wide and 48" long) that we used to surface manifolds and other parts that needed to have a flat mounting surface. Either option would beat ruining an expensive part with an attempt at doing the impossible.

You could also choose to use an original equipment manifold instead of a piece of rough-cast junk from China.
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!