This topic comes up sufficiently often to start a thread.
Rule number 1: THE CORRECT FLANGE OR MOUNTING GASKET FOR THE CARBURETOR IS ALWAYS THE ONE SPECIFIED FOR THE CARBURETOR BY THE ORIGINAL CARBURETOR MANUFACTURER BY CARBURETOR IDENTIFICATION NUMBER!
So the very first consideration is to determine the EXACT identification number for the carburetor. Once the identification number is determined, very easy to look up the manufacturer's bill-of-materials for the identification number, and determine the correct gasket.
IF the original identification is gone, then basically the owner becomes his/her own engineer, and would be responsible for disassembly of the carburetor, following all of the passages (circuits), and determination of the shape of the correct mounting gasket.
EDIT: Thought the above paragraph needs more explanation. The reason one MUST disassemble the carburetor and follow the passages, is that some manufacturers (Holley and Rochester in particular, but others as well), would use a "generic" throttle body (same casting number) and machine it many different ways for different applications. Often, one will remove the throttle body that has a passage drilled completely through the casting, only to find there is no matching hole in the bowl. One cannot look at the bottom passages alone, and determine whether or not each requires a slot to manifold vacuum.
Rule number 2: If the carburetor is being used for other than its original application, on other than the original manifold; AN ADAPTER AND A SECOND GASKET MAY BE REQUIRED.
One should ALWAYS follow rule number one, then place the correct carburetor gasket on the manifold, and check for irregularities. It may be a second gasket needs to be placed on the manifold to seal some passage. If this is true, for best reliability, a thin spacer or adapter should be fabricated. Then the carburetor mounting gasket would seal the carburetor to the adapter/spacer, and a second gasket would seal the adapter/spacer to the intake manifold.
Rule number 3: engineers generally know what they are doing.
Those who read my posts know that I have published on these forums listings of aftermarket Carter, Stromberg, and Zenith carburetors to be used on various Chevrolet engines BY engine displacement AND year! These lists have been gleaned by researching original documents published by each of the manufacturers, and should be used as published. These applications were tested in the original time period, and found to work. In other words, if one installs a Carter carburetor on an application from the list, then the correct mounting gasket will be the one specified by Carter engineers for the carburetor. If the enthusiast attempts to install this carburetor on other than the original intake manifold by displacement and year, then the enthusiast becomes the engineer, and is responsible for the installation.
So once one has determined the correct gasket, where does one find it? I don't have the answer to this question.
If one buys a rebuilt carburetor, then it should come with the correct gasket. If it didn't, shame on the vendor. If the enthusiast didn't ask, shame on the enthusiast.
If one buys a used carburetor to rebuild, then the correct gasket should come in the rebuilding kit, PROVIDED THE ENTHUSIAST SUPPLIED THE IDENTIFICATION NUMBER WHEN ORDERING THE KIT. Again, this would be the responsibility of the enthusiast to ask the questions.
And, as always, check the second and third lines in my signature block
Jon.
