Headers can be built specifically for a B or an RB:Case in point. I had a set of Pro-Parts headers that DIDNT fit a 383, but fit a 440 just fine. The 383 hit the header at the pan rail, the 440 didnt. So not all B/RB headers are the same, but should be! The flange MIGHT be specific if the kickout is less (smaller radii for the 90 bend out the port) for a 440 than a 383, that makes a little sense as the 440 head is farther outboard than the 383 and space may be taken into consideration when they design them (a larger radii is always better than a small radii when it comes to bends)