The EGR valve test from the 1985 factory service manual says to put vacuum directly to the EGR Valve. If the motor runs rough or dies, the egr valve is functioning correctly. They say if this does not happen, the valve must be replaced.
Are you certain that a header or gasket can block an EGR passage? If you look at the exhaust face of the header there is nothing putting out anything other than the actual exhaust ports that would go into the header. I just took another look at my factory manifold. No passages or anything for EGR.
Also look at this picture
I don't see anything related to EGR anywhere near where a header or gasket go. You can see in the bottom right corner of the head where the EGR passage is.
