This seems to be a common fault with exhaust manifolds [xx(]
They tend to distort and lift in the middle, this one was .060" (1.5mm) low in the middle. The cure is to build it up around the ports with weld,and skim it back to flat, like this one. This will give much more nip on the gasket and prevent blowing
"You,re only supposed to blow the bloody SLIDING doors off"