If I understand this deal. Your manifold inlet should be smaller than the 261 carb throat. This creates a "step" in the air flow. A 235 carb on a 261 manifold would seem better because the step does not block air flow. Folks have used this second scenario with OK results. Don't know if it would run OK or not. You can't swap, buy or sell on this forum. Use the proper forum on Stovebolt for that. Buy a 235 sized B1 carb and proper gasket and put it on. The RTV thing is NO GOOD either. Please post the part number of the gasket and insulator you got from Classic Parts.
Classic has two different carb gaskets with radial slots and two different insulators, they all say 1954, so you may have purchased the wrong ones for your setup....but what you are trying to do may be a bad idea anyway so wait for more info/advice.
Something is screwed up here. You say "holes match". Carb throttle bore and manifold inlet diameter? You say the manifold is 235. The carb comes back to 261. What "holes" are you talking about? Mounting holes or big hole in carb and manifold? The 235 and 261 carbs are 1/8" different ID's???????? (1 9/16, 1 11/16)
Carbking will straighten this out soon.
