The Holley 500 two-barrel is rated as a two-barrel, with more vacuum across it than the four barrel rating. I believe the four barrel rated at 1.5" depression and the two-barrel is rated at 3" depression. The correction factor is the square root of 2. So rated like a four-barrel, the 500 Holley is really a 354.
R.
This comes up about once a year.