I was told that Cyclone was the original manufacturer of the '65-'66 Tri-Y headers and glasspacks. I just looked it up and several sites are saying the same thing. Cyclone probably included the gaskets and bolts with the headers. Out in my garage is an original Shelby parts catalog with pictures that "might" show what the gaskets looked like. I think I even know where its at, in a file cabinet where I put it in back in 1977. If I can remember I'll look for it tomorrow. It looks like this one below. Somewhere I also have the list of who all the original parts suppliers were. I'm pretty sure I have a Cyclone exhaust catalog.
The modern composite head gaskets work real well, especially for aluminum heads on cast iron blocks. They are designed to handle the different expansion rates of the two metals.
Most of the aftermarket header manufacturers used a plain "paper" gasket back in "the day". Once you fire it up for the first time it really won't matter what might be printed on it, if anything. Also, FWIW, when installing "old school" paper header gaskets, lay them out submerged in the bathtub the night before and let them soak. Install them wet and tighten them down, then fire it up to "cure". They'll nicely seal to both metal surfaces.
As far as head gaskets, again...what does it matter? From a Concours standpoint, Ford painted their engines with the head installed and painted right over any exposed gasket so as long as the exposed areas are the same shape, easily referenced in photos, the old adage "what you can't see won't hurt you" would seem to apply.
I would also be concerned with original gaskets, they may contain asbestos and if I recall my father correctly, it was best to paint them with aluminum high temperature paint before installation for some reason?