I've used fiber header gaskets on FF1600 racing engines. They work IF your header flange is thick and flatflatflat.
My header was, of course, NOT. I began blowing out gaskets after a couple of seasons (the section right between two neighboring ports, where there is no hardware keeping the flange tight to the head, just like what we have here!) Your first thought is "it wasn't tight enough," so you crank down a little harder next time, which warps the flange even more, making the problem worse.
Old-timers advised me to use copper RTV on both sides. It didn't fix the problem, although it probably would have helped a lot if the flange had been flat(ter).
I eventually found out about Remflex gaskets. Super-thick graphite gaskets that really really squash. That was the last header gasket I ever had to buy. I can't recommend them highly enough. It paid for itself pretty quickly.
Remflex Exhaust Gaskets
Remflex Triumph L4 Exhaust Gasket Set
They also offer the material in bulk so you can cut your own.