I've run ported, at the heads, factory manifolds on a 273 with "J" heads. It ran great with either the single Commando straight through or duals with turbo mufflers. You will leave hp on the table but I got tired of dealing with headers a long time ago. You can run a 71 340 center dump exhaust manifold on the passenger side as it exits at the same place but has the bigger ports and cross section. I had mine Jet Hot coated inside and out. When I built the car TTI's or Doug's headers were not available.
