How much do I have to port a D-series header to make it match up right?

 

The sources I've seen indicate that approximately 1/8" of material needs to be ground down from one side of the header flange ports. The easiest way to do this is to place a ZC exhaust manifold gasket against the header flange, line up the bolt holes, and mark off where the material needs to be removed.

 

Thanks cool, i'll do that.