put some copper rtv on both sides of the gasket and let it set up. Also, retighten the header bolts after a few heat cycles. Tighten them center out. I use 1" long SS allen head bolts with lock washers on iron heads and never had one back out. ARP also sells good hardware.
the percy's aluminum ones are also awesome (and reusable). i went through 3 sets of fel-pros this spring when i re-did my exhaust (passenger side flange must be slightly warped), found the percy's at autozone and i dont think ill ever need to buy another set. coupled with some 3/8-16x1" long allen heads (easier to get around the primaries), and im in it less than like $40 and didnt have to mill my headers (or get new ones!)