I asked, on the Alfa Digest, for opinions on Shankle headers vs. Alfa European cast iron headers. Russ Neeley suggested that I use the header flange gasket to compare the shape of the headers.
It looks like there is enough material to grind on the Shankle header to smooth the transition from the head.
Tom Sahines wrote: “The only power difference will (be) evident over 7000 RPM. The Shankle headers will be significantly louder.”
The Shankle header is also (warning: PDF) approved in California by the Air Resources Board (CARB). It has the mounting holes for the air injection rail that is part of the pollution control air pump.
I bought an extra air rail to modify to fit on the cast iron header but that seems to be a back-burner operation.