[HO455]

Trading one new carburetor for another one is not likely to solve the problems you mentioned. There are at least a half a dozen things that could be causing your been poor performance. Many of them won't cost anywhere near as much as a new carburetor. And you may have more than one thing out of adjustment.

What kind of ignition do you have?
What is your timing set at? initial? Total? How much mechanical advance does your distributor have? At what rpm is the mechanical advance all in? How much vacuum advance are you getting?
How was the cam installed straight up? Advanced?
Was your timing tab zeroed to top dead center when the engine was assembled?
Are your spark plug wires installed correctly?
How were the valves adjusted?
Any of these things could be wrong and limit performance and changing carburetors doesn't cause them to magically get corrected.

Have you looked down the carburetor bore while someone floors the gas (engine off) to verify that the secondaries are actually fully open?
Before changing carburetors you should hook up an Air/Fuel ratio meter to find out exactly what is happening.