Thanks for the replies. I already concluded the manifold and cylinder head preparation was less than ideal. I suspect the surfaces had some
oil residue remaining prior to the application of an excessive amount of silicone as evidenced in the photos.
After removal of the old gasket material and a thorough cleaning of all surfaces I replaced the gaskets with FelPro 1250 S3 and used "The Right Stuff" on the China Walls and a very thin application of RTV around the water passages on the manifold and cylinder heads.