With the 235, would suggest the aftermarket YF which Carter made specifically to work on the 235. (See the thread 'Carter Chevrolet YF carburetors').
It makes good economic sense for a manufacturer to offer replacement units for the more common product, so Carter offered replacements for the YF replacements to replace the Rochesters on the 216 and 235. I have not seen any indication that Carter offered a direct replacement for the 261.
Generally, a manufacturer will have more resources to spend on engineering a replacement unit than the average hobbyist. When possible, I like to use a unit which has been specifically engineered for the application.
Jon.
