They sell headers that fit all the perfectly stock spec frames. As you know, all cars do not fit stock specs exactly. My car is an A body and I have had 1 5/8 and 1 3/4" headers for SB heads and 1 3/4" for W2 heads. Mine fit as spec'd. The problem is "Is the frame and mounts EXACTLY as speced?" A frame shop can tell you. My cousin worked in a body shop as a mechanic. Most cars do not match factory specs. Even the ones that had not been in an accident.