I am looking to install headers on my F250 and my Mustang.  I want to know really what effect the headers will have.  How is the power band affected?  Are they really aimed at only improving the top end?  How much sacrifice is there at the bottom end?

For the truck, the engine is a 390.  It is mated to a T18A 4 speed and a Gear Vendors overdrive.  The rear gears are 4.10s and the wheels are about 32 inch diameter (225-70R19.5).  I use the truck for camping, and I need it to carry a camper and pull a jeep.  The engine never sees rpms past 3000 for daily driving, and never past 4500 for short bursts.  I want to know if headers will help me or hinder my truck's performance.  Any and all insights you have are really appreciated.  

Well, just putting on headers will make some difference but not a whole bunch.  If the truck has a restricted exhaust - log type manifolds, single small pipe, etc. then they will help breathing.  They will also lean out the carb calibration, so you'll need to re-tune the engine.  Small tube headers are best for torque production.  I think you'll find most 390 headers to be 1 3/4" - not really small tube.  To get the most benefit from a set of headers, they should be part of a coordinated upgrade.  Intake, carb and headers are a combo that goes together.  For the RPM range you describe, and with no other modifications, a good set of dual exhaust with a crossover and decent mufflers would likely do as well.  

Thanks for the info so far....

A little more about the equipment on the truck:
The engine is a rebuilt long block with 130k miles.  I do not know the details of the cam, heads, or the valves, but I assume they are near factory stock specs.  The engine is fitted with a Holley 600 CFM 4 barrel carb and an Edelbrock dual plane manifold.  The ignition system is from pertronix solid state point replacement and matching coil.  The exhaust system has factory manifolds that feed into a 3 inch diameter single exhaust pipe.  The single muffler is a rather long affair with 3 inch inlet and outlet. The truck runs rather quiet, not a loud exhaust note.

I figured it would be smaller tube headers.  I just did not know if they would really bring much of an improvement to the performance.  Sounds like I might not gain much at all from switching.

Any more ideas....?