There is a difference between 261 Rochester carburetors and 235s, so if it is a carburetor issue, it's probably under powered. For a 1961 chevy 235, the main jet is .055. For a 1961 chevy 261 the main jet is .063. I put a 235 carburetor on mine to get it going, it worked ok, but accelerating off idle it would stumble and have a lean condition, would backfire. When I had it rebuilt the mechanic drilled the main jet out for me. Pic is from 1961 chevy truck shop manual supplement. yours is a bit older but should be the same idea.
