Take a piece of styrofoam packing, or cardboard, cut out to fit over the carb; jab a pencil through it so it sticks up beyond the height that the air cleaner would and place it on top of carb. Make sure it's not so long that it will poke too far in to the carb. Then close the hood. The hood will push the pencil down into the cardboard/styrofoam, raise the hood and measure from the bottom of the cardboard/styrofoam to the tip of the pencil that the hood pressed on. There's your clearance from the carb/air-cleaner mounting surface to the hood.
Redneck ingenuity at it's best!