I just pulled the headers of my '67 RS w/ZZ3 engine. The ports on the header appear to be a D shape and the Hedman header ports are circular. Is this a problem? I would think so, but not sure

If the header flange is covering part of the exhaust port I imagine it would cost you some power. You will likely have this occuring if you are using 1 5/8 primary tube headers with the "D" port. Since you have the header off of the car, make a template of a header port and put it up to the head to see for sure. There are some "D" port specific headers out there. If you can't find a set for your chassis, consider using 1 3/4 primary tube headers, as they will surround the exhaust port nicely.

Good feedback. The header port is slightly smaller than the D port. They are 1 5/8 headers but the immediate inlet measures right at 1 1/2. The manufacturer of the ZZ3 suggests using 1 3/4 which would sit nicely over the D port without restriction.