Headers vs 340 manifolds

Reading this again since I posted this question 2 years ago

Completely stock 340. 4 speed car 3:23 gears non sure grip

Thinking about long tube headers vs stock 340 manifolds.

Car will be tuned if header installation moves forward.

Realistically, how much gain can I expect?

No other modifications planned.

Asking all the Mopar experts on here.