From what I've been told the factory MLS head gaskets become the weak link at high boost levels. At 34-36 psi of boost the engine should be built with custom copper head gaskets. Not many are wiling to do this because copper head gaskets require the cylinder head studs to be re-torqued after the initial start up and heat cycle. Unfortunately these engines with studs in the Mustang require that you drop the engine to be able to do that.
Hopefully it is only a head gasket failure
Hopefully it is only a head gasket failure