Why would hot coolant-water be much better than exhaust gases, unless your intake manifold was a properly installed Offenhauser-style intake?
The Offenhauser intake allows the correct use of a dual-port bottom-plate (they are now hard to find - images are attached) : the plate allows an inlet port/fitting and an outlet ports/fitting - for two water hoses (properly plumbed to engine coolant ports)?
The Offy intake also allows flow from one side of the intake manifold to the other side of the intake manifold, through internal passages.
If there is no flow of exhaust gases or of coolant - neither heat-source is as good as having engine-coolant or exhaust-gas flow.
I ran such a "coolant flow" set up on a 261, until I worried about providing more opportunities for breaks in coolant hoses. I now have two exhaust gas lines attached to the plate under the Offenhauser intake. It has worked fine for a few decades.
I think someone posted long ago a description of an original Fenton set-up that somehow allowed exhaust gas flow for heating the intake?
