They allow you to use a later down draft carburetor. They are available new, though they pop up in ads and swap meets once in a while.
The use of a Stromberg or Holley carburetor originally used on the flathead V8 allows you to match airflow of engine to carburetor, both the A engine and the Stromberg 97 or Holley 94 have CFM flow rates of approximately 160 CFM. This balance yields good results and efficiency..the stock Zenith updraft flows at 80 CFM or so..