What they may refer to is that some heads do not have a boss and are missing one mounting hole at the outside hole for the header application to bolt to. There are adaptors for this and other ways around it. BUT, your 6X heads will work just fine... you have all the mounting holes.

So you can use any set of D port headers you can find for the 1st gen application. I use Hooker Super Comps, but they are not inexpensive!

Good luck - Brett! - 67 Drop Top 455!