"Has anyone ever seen a dual updraft manifold for any GM inline?"

I believe one of the big GMC sixes (maybe the mid-1940's 426?) had a dual updraft setup. My uncle used a retired GMC road tractor to haul BIG boats from his shop in Goodlettsville Tennessee to Old Hickory lake after he repaired them, back in the mid-1950's.
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!