Interesting how the Stay In Place Order can cause us to look in boxes and nooks and crannies of our property. The strange thing is that as I get older (north of 60) I remember the stuff I acquired 5 years ago and going back. But, a lot of stuff I've acquired relatively recently (5 years or less) I'm like, "When did I get THAT?"
Cast exhaust manifolds use to be a dime a dozen and you couldn't give them away for free. But today people want them for originality (if you have the right part #), engine swaps, turbo builds etc.. So, the few that remain have gained value. Honestly on a street car with mufflers I doubt just how much headers help in the typical driving range (idle to 3,000 RPM). Years ago there was a somewhat mild Mopar 360 build where they tested everything from the lowest of the low (318 cast manifolds) to $800 TTI headers. The difference..., 10 HP at 5,800 RPM. And that was without mufflers. Throw mufflers into the equation and it was probably even less HP gained.
I always found it "apples to oranges" when a magazine in the 60's/70's would take a cast manifold, small, single exhaust, choking mufflered car and convert to headers, dual exhaust, larger diameter pipes and turbo style mufflers and then claim that headers added all the power.