[Christ]

Use the header and the down pipe, mate it up to the smaller exhaust with a taper section.

Exhaust without intake mods just makes it easier for the exhaust to get out, and even if you mod both intake and exhaust, it doesn't "let more air in" until you put your foot closer to the carpet.

The only reason not to direct the gasses out in the most efficient way possible is if you have an engine that relies on residual exhaust to pre-charge the cylinder to limit intake charge.

You don't, that I'm aware of.