I have a 67 Firebird 400 with D port heads, the stock manifolds are pretty much done and was looking into headers, Doug's, Thorley, seem about the best but I'm also liking the Sanderson block huggers, I would prefer Tri Y but seems no one makes them, trouble is there seems to be a complaint about every header and fitment. I've done headers on other vehicles his time I'd rather do some research.