My only advice is to bolt them together and have them milled to a common surface, otherwise your in for a bad time getting them to install and seal to the head. A lesson many of us has had to learn the hard way.
Does the Fenton or Williams dual-exhaust manifolds/headers directly attach to the intake manifold? The Fenton headers that I have do not. No need to bolt together and mill, I think?
As Cletis describes, you can heat with engine coolant-heat or heat with exhaust-gas heat. I have used both methods - both methods work fine. I now use the exhaust heat method because there is no risk of coolant leaks.
Here is the simplest exhaust heat intake manifold plate for use with Fenton headers. It just uses one line; use a brass pipe plug in the hole in the other header.
All you need is enough heat to keep the gas from condensing.
