Don't waste the $ on the headers. It takes too much time and work and/or costs too much in labor to justify spending the $ for the small gains. The stock heads don't flow well enough to utilize the headers.
A high Flow H-pipe will be the best bang for buck. That should add anywhere from 8-14hp. A good set of mufflers, or a full cat-back may help to free up another 2-4hp.
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1997 Thunderbird 4.6L: Dynomax cat-back, MN-12 perf. offroad downtubes and Y-pipe w/ Magnaflow resonator, 80mm MAF with cone filter, B&M trans cooler, Inject-Tech chip, KVR front brake upgrade. 15.2 @ 91 before exhaust. Not bad for a 3800lb auto 4.6L
No Longer own: 91 LX 5.0L 5 spd
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