Air bleed in the center of the x has been there a while. The 600 has one on my 323 mopar single plane . Hollies need to make sure they can provide the correct fuel at idle. Some idle bleeds are too small for modern fast ramp/high lift cams.
Hollies are hollies. Fine for the drag strip or WOT. Cruising the perferred unit is the Q-jet or AVS/AFB. The needle obstruction in the jet really increases performance down low, stabilizes even fuel flow, and stimulates velocity for awesome acceleration. Right from the hit.
If you have a 350/383 the 750cfm carb fits perfect. Be it a 750 QJet or 750 vacuum secondary holley with stiff spring ,or 750 edelbrock. Out of the box they seem right. You never use the full cfm in the secondaries anywho. The performace comes from the primary side, the secondaries help past 2/3 throttle, and then do not flop open. The engine pulls against the spring, in a struggle to get enough air to meet the fuel ratio.