Tony - obviously, you have NEVER put any gasoline in the gas tank, therefore the carbs cannot leak! :p
The early Holley 1904/1920 (1952~1961) used a glass bowl (more rigid than metal). This was discontinued in about 1962 due to breakage from clumsy mechanics, and the commercial rebuilders started replacing all of the glass bowls with the zinc alloy version. The Holley 1904/1920 suffers from one of same maladies of the Rochester B (insufficient screws attaching the bowl to the body). Over time, the castings warp, and leaks occur. But glad you are happy with your Holleys, just don't take them for granted. The glass bowls are less prone to leaks than the metal ones, as the glass won't warp, but the metal body still does.
And you are absolutely correct concerning matching carburetors. This is a area where many don't understand that specifying the type of carburetor does NOT specify the internal size or calibrations.
Jon.
