Good point mope. Our Midwest Firebirds president sent us a recent email with this picture from HPP to show what the 326HO owner is looking for:

If the HD option on the '67 326HO is like the one for the '68 400's, the part numbers only meant a different sticker on the outside to show the HD filter element with the foam wrap. The other part numbers options for different emissions requirements might be plumbing differences?????