IIRC the gasget sets i used on small block Chevies had a little piece of
tin plate that clipped in place over the gasket. They had a small hole in
the about 10 - 12 mm dia so some heat could transfer to the carb base and
you could block off either or both ports.
I think a new set of gasgets would probably be in order anyway although
you could try and reuse them & silicone a piece of tin over the ports to
hold it in place. That won't cost you anything but a bit of time.
I think Mondello or some restoration outfit sells some sort of metal alloy
you heat up & pour into that passageway.
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