Patrick, the 261 runs the same intake and exhaust manifolds as a 235 so swapping carbs might not produce a noticeable gain in performance RPM-wise. Finding one that's jetted for a similar cubic inch displacement engine would probably improve the low end and midrange performance, or at least provide a good starting point. The 261-specific Rochester B is a scarce item, and I'm sure it would have all the characteristic Rochester headaches as well.
Jerry

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