FWIW I was told by a reputable carb builder that 3/4" above the vent tubes is all that is required. I wonder if there's both a minimum clearance above the vent tubes and a minimum clearance above the venturis needed to ensure sufficient airflow... 
2021 Challenger 6.4L Scat Pack 1320
100% stock: 1.680, 11.894 at 113.75 (DA 175 ft)
weight reduction, wheels, tires, Hellcat air box: 1.661, 11.686 at 115.97 (DA 710 ft)
1973 Challenger 452 ci street/strip [2008]
pump gas, DOT radials: 1.454, 10.523 at 126.44 (DA 514 ft)
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