On our 303" single carb gas engine (Keith Dorton-built from Hendrick SB2.2 bits and pieces plus his own ideas) the standard "dyno header", a Hendrick Cup tri-Y style, made 7 lbft and 13 hp more than the 4-1 headers we built for the rear-engined roadster. The 4-1 exhaust had quality merge collectors, too.
I would have built tri-ys, but didn't have the time (and maybe not the space without some creative work and aero compromises). I'm no header builder or designer, I just used the best info available at the time without getting into full design program. The 13 hp (which occurrred at a peak 100 rpm higher than the 4-1s) is an almost 2% increase in power we don't have available currently. Primary sizing and the sizing of the "nozzles" in both the primary and secondary merges is critical to maximizing power; time and money (and time IS money) will dictate your approach.
That said, your header guy doesn't sound well enough experienced in design (like me!) to perform your build unless you choose to overlook the possible power sacrifice. vic