Back before the days of fuel injection when they used floats inside the carburetor they wanted the carburetor to sit parallel to the ground standing still.
You will notice on the old intakes some of the new ones also, that the front end is lower than at the rear of the carb mounting pad. This really shows the angle of the engine and transmission as mounted in the car.
I have not noticed with the kegger intakes there being that angle that the old manifolds had that mounted carburetors.
