What is the best valve cover (top cover and side covers) sealant for a cork gaskets on a '79 MGB?
Moss sells a thick cork valve cover gasket that works very well. Secure it to the valve cover with "The Right Stuff" or "Ultrablack" both by Permatex. Then LIBERALLY paint the underside of the gasket with #2 lithium grease where it meets the cylinder head. This will allow the valve cover to be removed multiple times without damage to the gasket.
Moss sells a thin cork gasket for the very early MGB and the MGA three main engines. Use this gasket on the front side cover. Again, use either of those Permatex sealers -- but this time use it liberally on BOTH sides of the gasket.
The rear side cover receives the reddish/orangish rubber gasket. No need to use the sealer between the cover and the gasket, but certainly use it between the gasket and the cylinder block.
Hope this helps!
John H Twist