I haven't seen a good way to block a heat riser passage in the past 50 years- - - -including that tiny piece of stainless steel sheet metal that some of the hotrod places sell- - - - -it will burn out in a matter of weeks at the longest. I've even seen people pour the heat riser passages completely full of molten aluminum- - - - -it disappeared in less than 3 months of street driving. If you really want to eliminate that passage, buy an intake manifold without one- - - - -and trailer your car to the race track!
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!