Take a look at the thread title "Zenith physical carburetor sizes".

Reference is made to S.A.E. sizes.

There are other threads stating the S.A.E. sizes on Carter and Rochester.

Basically size 2 is 2 11/16 center to center; and size 3 is 2 and 15/16 center to center.

The above is true for Carter, Holley, Rochester, Stromberg, and Zenith.

Jon.

Good carburetion is fuelish hot air
The most expensive carburetor is the wrong one you attempt to modify.
If you truly believe "one size fits all," try walking a mile in your spouse's shoes!
The Carburetor Shop