A thin steel plate over the openings on your intake should do it. I don't know if they sell plates for that or not. Maybe use some gasket goop or make a custom gasket for itto make sure it is air tight.

I don't think the EGR valve will affect your idle or anything with the engine. Its the PCV valve that you don't want to get rid of, but that has nothing to do with the exhaust.

If you have an oxygen sensor, your car will need that hooked up again. The other fix would be to try to use an older computer from a non-California 280Z 1975-76. (?) I haven't tried it, but its an idea.