The "Soup That Chev!" 1955 Hot Rod Magazine article on "upgrades" to a 261 shows results for the addition of the 235 Corvette's dual-exhaust manifold resulting in another 5% increase in horsepower.
The addition of dual carbs resulted in a total increase in horsepower of 16% at 2500 RPM and 18% increase at 4000 RPM.
30% increase from a 235 when adding Fentons alone is unlikely.
Adding a higher-lift cam (261 cam or higher lift), more carburetor CFM, and dual exhaust manifold does give a noticeable increase, but maybe not 30%?
