Bazza's 318 is running (!!!), but....

To clarify, you would not have an exhaust leak there, only a vacuum leak. The carb and choke are obviously part of the induction system, not the exhaust.

The choke is of course regulated by engine temp. The choke well is just a closed chamber that provides a relatively accurate place for the choke to ‘read’ the engine temp. Can’t imagine there is a rust hole in it without other parts of the intake having similar problems.

The choke mechanism is operated by a bi-metallic strip. When the strip reaches xx temp, it opens the choke since it’s no longer needed once the engine is warm enough.

Regardless of whether the choke is working correctly or not and if you think you have a vacuum leak there, check the choke base and/or carb base and gasket, maybe something got pinched during installation. Also make sure there are no open vacuum ports in the carb base itself.

Could easily be an intake gasket leak too.