I remember a magazine article years ago they tested a 300 hp 360 magnum crate engine (factory 360 magnum with slightly smaller cam and M1 dual plane intake) and all the manifolds from the 273 318 340 and magnum all made right around 300 HP with maybe 10 HP from the best to the worst and the shorty headers were slightly better with the long tubes being the best by quite a bit (around 320 HP). Surely someone knows about the article... I would run whatever I have laying around unless I am building significantly more HP than that.