Mount a couple of 12 volt marine bilge ventilating blowers ahead of the manifolds, and let the air they provide blow across the manifold/exhaust pipe area. It will help if the intake air to the blowers comes from beside the radiator where it's several degrees cooler than the air behind the core support. Angle the blower outlets so the air is blown at a downward angle as it passes the manifolds.
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!