Another positive response for manifold heat for the Offenhauser. I also purchased my Carter/Webers from Langdon who at the time, recommended water heat. I chose to go with the exhaust gas instead.
My rationale was for ease in installation and reducing the potential for problems should my system develop a leak. With a water leak, you have the potential for engine damage if you don't catch it before the engine overheats. With the exhaust heat, there is no major effect other than a bit of noise. I have pictures in both my GM webshots albums that show the simplicity of the exhaust heat plumbing.
No "valving" is necessary as the firing of the engine creates a pressure differential which supplies the heated air to Langdons heat plate option. The heating process is instantaneous and obvious when checked with an infrared thermometer. Disconnecting the tubes left me with a rough idle condition and poor performance at WOT. Conclusion: go with heat!
Dave
