The V8 Owners Forum -

what sort of gasket is better for the inlet manifold, tin or composite??

Personally I don't think it matters, Never known of any inherent problems with any of them. It could be said that the compost ones give slightly better heat insulation. There at least 2 different comp gaskets as well. A compresses card type stuff with metal in the middle and a kind of rubber foam coated metal one. The card one is a good bit thicker than the others.

If you use a much thicker gasket than designed, wouldn't it create a step in the ports? Or would this be a good thing?

That's got to depend on how good it was to start with I guess? Difficult thing to check without an endoscope.
I was told the LDV V8 was better with the tin valley gasket, as it let the heat out. I'm not suggesting its more than a few watts, but somtimes thats all you need to avoid over heating