Running a 216 manifold on a 235 will require a set of stepped-down alignment rings since the 235 has larger intake ports. Some of the parts vendors who advertise here have the adapter rings you'll need. Running without the rings almost guarantees a vacuum leak due to gasket misalignment. Running a 216 carb on a 235 will result in a rich mixture at part thottle and midrange, and reduced HP at wide open throttle due to the smaller venturi diameter of the 216 carb. A 235 carb has a wider bolt spacing, so swapping a 235 carb onto a 216 manifold requires some machine work to relocate the studs. It's simpler to run the manifold the engine was intended to have.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
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