Jeff's car is using custom made super short air horns. The reasoning behind this is that the amount of space above the air horn (head room) is just as important as the horn's shape. There is only about 1.75" inside the air box to work with, so these horns were made 0.45" tall, leaving a head room of about 1.3" See them here:
Weber 44IDF Air Horns
For shape, you certainly want a fully radiused horn. Most horns are poorly matched to he carb throats, blending is ideal, or even the type of horns that are machined to go into the carb throat behind the choke.
Also, you want your air correctors to be at the same height as the air horns. Some of them are buried down in the carb, others have extensions so they rise up off the carb body. Its a slight design change and the effect is small but having the air correctors at the same pressure as the throats is ideal.
Length of horns has nothing to do with increasing airflow or reducing restrictions. A long horn flows just as much air as a short horn. Where length is important is with tuning your intake runner length for your engine. This is why there are hundreds of different opinions for hundreds of different engines explaining that X engine made Y more power with Z air horns but putting Z air horns on a Fiat motor may not be the best solution. Where do you want the power to be, up high or down low? Do you have carb spacers? All of these factors are important.
Finally, the number one thing, get cool air into those carbs. If you're sucking in engine bay air, you're leaving power on the table. Not only are you tuning your carbs for hot air, you're tuning the ignition and everything else for hot air.
