Use a piece of aluminum plate about an inch thick, sized to bolt to the bottom of the intake, and drill it for a couple of pieces of 1/2 inch stainless steel tubing. Plumb the tubing to the ports in the Fenton headers so exhaust gas can circulate back and forth through the plate. You can't plumb the exhaust gas directly to the aluminum- - - -it will erode very quickly. Drop me a pm with your email address and I can provide a sketch with the basic layout of the heat plate. Water heat is a waste of time- - - -when you need manifold heat the most is right after startup- - - -when the coolant isn't up to operating temperature.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!