I believe that the bolt pattern for the heat riser flange is also different between the early and later manifolds. Here are links to images of the heat riser flange gaskets in the Chevs of the 40s catalog for the early (216, pre-1950 235) and late engines. Has anyone ever attached a late model exhaust manifold to an early intake manifold? All that i have put together have been late 235s and 261s.

Hoyt