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I can't offer a real answer to your question, but...
20 years back I stepped up and installed a new set of L&L fenderwells on my '78 F-250 4x4 that I had swapped a 460 into.
A week after the install I retightened the headers. That was the last time I touched them. 20 years with zero header leaks is priceless.
In my opinion I would think that if the headers were in good condition with no obvious damage they should be worth 1/3rd of a new set. A lot would depend on age, etc.
FWIW.
When I installed mine I wanted to go with in frame style but had to settle on the fenderwells. My truck has a D-60 front end and the pumpkin would contact all the different L&L in frame conversion headers offered at the time. Perhaps L&L now offers an inframe that clears the D-60 pumpkin.
Rezvani's Latest Post-Apocalyptic Monster Is a Ford F-150 Raptor Underneath
Slideshow: Called the Fortress, the 850-horsepower pickup combines Raptor underpinnings with military-inspired features, survival equipment, and a starting price of $285,000.