273 Exh Port MisMatch

C130 Chief said

The early 273 manifolds will hit on later heads causing #7 to leak. The long outlet hits the leveling boss on the edge of the cylinder head down by the lower right hand head bolt. This keeps the port from sealing. And cranking down on the bolts will just break the manifold. Not saying this was a factor in your situation, but it's worth checking.

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And whatever you do, don't try to grind the extended port flange on #7 to get clearance, there's a water jacket right under the surface. I trashed a perfectly good 308 head that way. The only safe way I've found to do it is to mount a bare header flange as a spacer between the manifold and the head.
FWIW, I've run into this same type of interference with left side early 340 manifolds on Magnum heads, too.