My Son had a '65 Olds with a 425, which I think shares many parts of your 330. We never could find headers for it. Hedman shows headers for 67 up 350, but not your 330.

We finally just had a free flowing dual setup made with turbo mufflers. Somebody may make them, but I don't know who they are. You might start contacting all the major guys like Headers by Ed, Hedman, S and S, etc.

If worst comes to worst, the duals are really ok for this engine, and cast manifolds tend to be better street units anyway.

Don