You don't need headers to make power. Will you (usually) make more power with headers and exhaust? Yes. Do you need the difference? In street driving, usually not. I have one of each, both are stroker 340's, 422 and 426 cubes respectively. Both are built very similarly, similar cams ported iron heads that flow at around 280, both in 71 A-body cars with manual transmissions and 3,91 gears. Both have both been dyno'd so I have real data on them. The 426 cam specs are attached, the 422 was a very similar grind. The 426 made 450hp and 500tq. These numbers were made with exhaust manifolds, stock iron intake, stock Thermoquad carb, and everything else externally stock down to the belts and hoses. Now, the 422. It has TTI headers, mandrel bent 3" exhaust, a single plane intake, a Holly 950HP carb, 7AL ignition, and all the other 10 cent tricks. It made 530hp and 520tq on the dyno.

So, bottom line, A LOT of $ and optional equipment (as a package that works well together) resulted in roughly 75hp and 20lbs of tq most things being equal if not at least similar.

On the street you simply don't need the extra hp. the 450 engine will blaze the tires at any speed and torque is so close it feels the same.

.02