Don't see the big advantage there's probably a 5-7 hp difference, there's a 4 hp difference between 318 to 340 manifolds don't see 273 being much worse than 318 and magnum being that much better than 340. There is another shoot out which I can't find now that the best combo is one side 340 and the other magnum, forget which one's though.
Some Headers are arguably worth it on a 300 ish hp engine.
Installing Exhaust Headers Into 300hp Crate Engine - Mopar Muscle Magazine