I have worked on several OEM stock 340 six packs, they came lean from the factory and needed jetting to help them run as good as they can at WOT.
How does your car drive with those plates in the carbs?
I have seen a lot of those type Holley metering plates converted to removeable jetting by drilling and tapping the bottom fuel inlet holes to #6x32 thread size and then use #6x32 blank brass set screws and drill them to the size (with a good quality numbered drill set) you want
Don't change the center carb jetting on the 340 carbs
IHTHs