I put a Borla header on my Jeep (inline 6, not V6) and it improved the
performance and the sound.

If all you want is sound, you do that with a new muffler.

You need to find headers that will pass the smog requirements. This
generally means that you have to be able to screw in the O2 Sensors.
Companies that sell headers have to inform you if they are street legal or
not. Street legal headers can pass a smog test ...

I recomend the ones that came on it.
They are cheap, will last forever, won't blow gaskets,
and already installed.

There was a time when they designed exhaust manifolds merely to get the
exhaust out of the engine. The main goal, it seems, was to make the corner
from a relative vertical path to a horizontal path. The exhaust manifold was
not very efficient, and swapping it out for a header improved exhaust flow
immensely -- creating power and economy at the same time.

In a world of increasing CAFE standards, the design teams have managed to
make the stock exhaust manifold nearly as efficient as it can be. This means
that adding a header is not likely to produce significant results in terms
of power and/or economy. You may eek out some improvement, but you have to
figure the Savings Per Mile then calculate the number of miles you have to
get under your belt to break even. If you saved a penny or two per mile on a
$500 set of headers, it would take somewhere between 25,000 and 50,000 miles
to recover the cost.

I would suggest you can improve the airflow through the motor with the
muffler alone. Or, the muffler and an increase in the size of the exhaust
pipe. Bumping the exhaust system from 2" to 2.5" will cut the back pressure,
improving the flow and changing the note (sound). There is a point where the
backpressure decrease (exhaust diameter increase) actually harms
performance, so you have to walk a very narrow line.

I would steer clear of the national chain stores for my muffler needs, and
visit a shop that does performance upgrades to get advice that fits your
truck and your checkbook.