Just to update a bit, I took my 67 Barracuda to the strip a couple of months ago. I went from 13.70s at 98 with stock 340 hi-po manifolds, 2.25 exhaust and 3.23 gears to 13.07 at 106.5 with TTI headers, 2.5 inch exhaust and 3.55 gears. Both times through the mufflers. With a pretty dinky cam (.450/.455 lift).
That's a 3/4 second improvement from headers with a virtually stock 340 (Duster) and a very mild 360 (Barracuda). Not sure why anyone would be skeptical of a half second improvement with headers over exhaust manifolds with a stock or near stock motor.