headers vs manifolds on a 318 2bbl difference ????

When you've installed dozens of sets of headers, told the customer you're going to have to tune the engine, jets, plugs, timing, and they come back with a big grin, take it to the strip, and improve their time 2 - 4 tenths, then suggest they open the headers for more, - and they get better times, then specifically the 340's, - tell them you want to replace their AFB/AVS with a 3310/780 Holley, and if they don't like it, I'll put their old carb back on, ALL absolutely free for the price of the carb I got for $50 - $80 from a swap meet, kitted and other, - if they kept it.
They come back with a huge **** faced grin.
Have never re-installed the old carb, never, - over a dozen times .
All without a Dyno , just experience.
Good luck .