That thermovalve isn't open to the intake manifold; it just blocks/opens (at a certain coolant temp) passage of vacuum between the two vac hoses normally attached to it. With the stock Mikuni carb, I'm not sure how well it'd run without the thermovalve operational, or whether that'd do better with those two hoses bridged together (or replaced with a single continuous hose) vs. blocked/capped off at their source.

A Weber swap wouldn't use the thermovalve at all, so you'd only keep it in place to cap the coolant port it screws into, or you might find a BSPT 3/8"-19 TPI threaded plug for that. When removing the Mikuni, you'd remove the carb along with all vac hoses attached to it and the control box bolted to the driver-side inner fender. Then with the Weber you'd really only need two vac connections: the fat one from the manifold to the brake booster, and a smaller one from the carb or manifold to the distributor vac advance capsule.