Another option would be to go old school and split the original manifold. It's not all that difficult to weld in a blocker plate between the front and rear sections of the exhaust manifold and add a second exhaust flange to the front section. It solves the manifold heat problem by using the original heat riser valve, and it looks period-correct when somebody looks under the hood.
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!