MainJet,
All 216, 235, and 261 intake manifolds use alignment rings between the intake manifold and the head. There are two different sizes (
look at Jim Carter\'s listing). Some people say you can get it to work without rings, and some people say they have mixed and matched the older/smaller port piece with the newer/larger port piece. I've never done that and would try to avoid having to do so.
The carburetor mount is also different for pre-54 (216/235) intakes vs 54-62 (235/261) intakes (2.5" vs 3.0" carb mounting hole).
Tim
