I am running exhaust heat to my manifold for about 3 years now and it works very well.
We get triple-digit and freezing temps alike and I don't have to use carb insulation spacers with the aluminum intake either.
The exhaust DOES flow through the manifold bottom, because only one exhaust valve is open at any time.
A small portion of the exhaust pulse can flow and exit into the other header, which has all exhaust valves closed at the same time.
There is actually one vendor who sells a heatplate with only one connection - no flow there....
