Thought this would be interesting since I saw some posts about a 454 swap recently. In this month Chevy hi-performance the took a stock 454 and dynoed it with cast iron manifolds dual 2.25 exhaust and generic turbo mufflers. Results 450TQ @3400-3600RPMS, 404HP@5600RPMS
Then they bolted on a set of hedman 1.75 headers, dual 2.5 borla pro-xs muffler. Result 500TQ@3600RPM 432HP @5400RPM.

While a nice gain its really only 8%. When compared to the cost of headers for a bbc in a 3rd gen and the PITA factor maybe not worth the time. Just some info for those who have to consider this in the future.

 

actually the hooker super comp long tube headers for a big block'd 3rd gen are cheaper than the small block super comps.

 

The 28 hp might not seem like much but 50 pounds of torque is a lot. There could be a lot more potential but they only used a larger exhaust. Most headers like that will have a 3" outlet and they went with a dual 2.5" exhaust system.

 

Yeah, that is a lot of TQ gained there.

Plus, if you starting boltin on mods that need the engine to breathe, like cam, heads, intake, the manifolds will hurt you even more.