Kim,
Is there any possibility that your annealing method brought some area of the gasket to partial melting and, theferfore, surface deformation? If re-torquing the exposed four bolts does it, count yourself lucky, but I suspect a new gasket or, at least, re-annealing will be called for. Have you done a compression test? Maybe both sides are leaking. Before and after tests could tell you if the re-torquing did the trick.
Richard L.