It (literally) sounds like you are real close. Try the thick gasket set (what is this - where do you get it?). The original gaskets were thicker than the Fel-Pro gaskets that I have seen (I have an old NORS set of gaskets that are much thicker than modern gaskets).
I have also used a double thickness of manifold gaskets with good results (with and without using copper-colored, hi-temp, RTV sealant). Some might say that this is an admission or error or defeat - I say if it works, go with it.
The other "final solution" would be to have the pair of manifolds planed/ground to be flat (I do not know what this process is properly called).
