On 216/235/261 engines, the Offenhauser intake and Fenton headers had to be ground in order to be attached together (their bodies rubbed/interfered with each other).
That intake/exhaust interference grinding was in addition to the grinding needed for head-mounting clamps to line-up on both manifolds.
On a second installation, I ground the bolt-clamp faces (on one side of each clamp) to produce a tight head-mounting.
