The heat riser valve promotes quick warmup by directing exhaust gas against the bottom of the intake manifold when the bimetal spring on the shaft is cold. As the engine warms up the spring heats up and allows the counterweight to turn the flap valve to direct heat away from the manifold to prevent vapor lock, etc. with a hot engine. Removing the flapper valve will result in slow warmup and rough running when cold, and possibly vapor lock and percolation of fuel out of the carb float chamber due to overheating the carb on a long run or a hard pull with a heavy load. It's better to fix the heat riser or find a manifold with a good one. A 235 exhaust manifold should work with no problems.
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!
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