Headers vs 340 manifolds

headers on a stock 340 through mufflers aren't going to gain anything noticeable.

Absolutely 100% wrong .
I have had tooooo many customers come back with the biggest **** faced grins after their first drive with headers.
They certainly noticed, not just one, or a few, - - dozens.
Then they go to the strip and open them up and gain up to 5/10ths, or can't get traction .

Any breed.
The power isn't just at wide open throttle, it's at idle, it's at slow cruise, hi cruise, the extra power you don't notice is just less gas pedal thru all driving conditions.
The low back-pressure, scavenging, equal length tubes, doesn't only work at wide open throttle.
Those gains are there in all conditions, otherwise all cars would come with log style exhaust manifolds, no exhaust direction from the head at all, yet they made cast hi-perf exhaust manifolds with direction, then tube headers, all improvements, at a $$ cost.