I would use them Stu and not worry about port size in your case. It is not that big of a deal with our street cars.

They are very tuff gaskets and can be reused many times. You know if your heads and header flanges were flat, then you would need no gasket.
I have used them on every race car I have built, but not on a street car. I think they will seal fine and last forever if your flanges are flat.

Suggestion... At least hand file your exhaust flanges and your heads flat after coating your headers to insure sealing.