In the "For What Its Worth" category:
Over the last 5 decades we have sold thousands of carburetors.
The "rule of thumb" we use when suggesting a different from stock ORIGINAL EQUIPMENT carburetor is that the "donor" engine should be within three (3) percent displacement of the "donee" engine.
So three percent of 261 is 7.83 (round it to 8.0 for ease of calculations).
Thus, by using our rule, a real carburetor calibrated for a 261 could probably be successfully used on engines with similar RPM requirements from 253 to 269 CID.
Whether a Rochester B would work on any of the above is subject to discussion
Aftermarket carburetors are deliberately calibrated too rich, so the above rule of thumb does not apply (well, it does, but the percentage increases slightly).
Jon
