That was a chronic problem with those carburetors. The passageway in question carries exhaust gas- - - -it's part of the heat riser system. A leak there will put exhaust gas back into the intake, resulting in a condition like an EGR valve on a later-model engine stuck wide open- - - -lousy performance and an engine that won't idle. CAREFULLY drill and tap the rotted-away mounting studs, and draw-file or machine the gasket surface flat again. I'd suggest taking the manifold to a machine shop where it can be set up in a Bridgeport mill for both stud replacement and fly-cutting a new gasket surface. Draw-file the throttle base dead flat before reinstalling the carb, too.
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!