Not saying if you put Magnum heads on a 273 your gonna gain 100 hp, but couldn't see less than 50-60 . There is dyno numbers for 5.9l Magnum and LA 360 both with 4 bbl and LA 360 exhaust manifolds, LA 360 made 266 hp @ 4300 rpm and magnum made 314 hp @ 4600 rpm. The only one I'm guesstimating is a 360 with 273/318 heads.
Installing Exhaust Headers Into 300hp Crate Engine - Mopar Muscle Magazine
https://www.hotrod.com/articles/mopp-0105-dyno-test-360-smog-motor/
