You have either one carburetor too many, or one not enough. Inline sixes with three siamesed intake ports run best with either one, or three carburetors. The simplest, least expensive way to solve the problem is to go back to what the Chevy engineers decided to use 70-something years ago. I'm building a custom intake setup for a 261 engine that has three Keihin side draft carbs originally used on the Harley Davidson EVO engines.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!