cars runs good, but exhaust manifolds get cherry red block is much cooler

The exhaust manifold does get very hot but if it is glowing cherry red it could be a restriction in the exhaust/ restricted catalytic converter, it could be a lean air fuel mixture, or retarded ignition timing all of the above should cause a loss of power or fuel economy.

The only reason the exhaust manifolds can be turning red - when everything else is working properly - is that they are made of thinner metal and not the solid cast iron used in older manifolds. Thinner metal cannot conduct heat fast enough. Make sure nothing that can melt is near and definitely not touching the manifolds.

I agree with the lean fuel mixture or cat converter plugged. I have seen both cause this issue, with the majority being converter.