You might think I'm crazy but I've always used stock manifold gaskets with my headers and have never had any leaks. A oldtimer auto mechanic told me this trick so I tried it and it works great.
Stock manifold gaskets do not separate the center ports. When I installed the headers for the first time the gaskets that came with the headers had been folded and were damaged so I used the stock manifold gaskets that came with the Fel-Pro gasket set. Because they didn't have the center web the leaked like a sieve.
Most all headers eaders have a raised bead around each port and with out that center web in a stock manifold gasket you have an open path for a leak. They may work on headers without the raised bead but can't seem them working well on headers with them.