I think if it was me (and it might be) I would buy the cheapest set of b-body big block headers I could get, uncoated, and do whatever bending, beating, or modifying that needed to be done to get them to fit..... then send em out to be coated.
I have a pair of custom made 440 a-body headers.... but they require butchering the inner fenders, and they are too big for anything less than a 700hp 540.
