The carburetor flange shape, size, and bolt pattern need to match the intake manifold. If not, they won't bolt up without an adapter. There are three main 4-barrel flange styles.
Spread Bore
OEM style carbs usually have a spread bore design. Spread Bore Carburetors have the smallest bolt pattern of the 3 styles.
Square Bore
Square Bore Carburetors have a larger bolt pattern. Most aftermarket carbs use a square bore flange.
Dominator
Dominator Carbs have the largest bolt pattern. Dominator Carburetors are only used on high performance engines.
Carburetor Adapters
Many aftermarket intake manifolds have a dual pattern flange. That means it will mount either a spread bore or square bore carb. If your manifold does not have the right bolt pattern, Carburetor Adapters are available. Adapters allow a carb with one flange style to bolt to an intake with a different style flange.