Headers vs. manifolds

Theres's a lot more to headers than a modest increase in horsepower. Their horsepower-effectiveness depends a lot on the rest of the combo. They also give a substantial boost in low-rpm to midrange torque, and that translates directly to mpgs. In your 73 you might get away with cheap under steering-link headers, that often barely last two years. Or you can get the good stuff that will last 4 or 5 times as long, and you never have to worry about speed-bumps again. And the thicker gauge of the good stuff has the bonus,IMO, of not sounding like a cheap,tinny,loud,header.
If by warmed over, you mean a cam over about 225* degrees,or one with over 50ish degrees of overlap, you need headers. Even factory 340s responded well to headers, yet they had just 44* of overlap. Even my factory stock 73 teener short block, liked headers, with nothing more than a 4bbl.
Magnums have nice heads;with a hot cam, IMO, you would be choking them with log-manifolds.
But then again the 340 did pretty good,in stock trim.