A question on holley carbs

I'm sure it is an engineered feature, not a flaw.

What is the rest of your combo?

I'd just set it and see if it runs correctly. I never set it based on the slots. Screw the mixture screws in all the way, then back each out 1-1/2 turns. The back out the idle speed screw all the way, then in just enough to where it moves the throttle blades off the closed position. Bolt it on, set the initial timing, then crank it and set the idle where you want it using the idle speed first, then the mixture screws. If it doesn't run that way, then you may be correct in your suspicion about a problem with the slots. I'd also remove the bowls before bolting it on to insure there's no trash in em. And if it's sat for any length of time more than a few weeks, you'd be doing good to replace the powervalve and the accellerator pump diaphram before using it.

The 570 is only going to limit the topend on your 331. My 331 is a bit wilder with Canfield heads too, (might be the same as yours, I don't recall the model numbers) more cam (Ford Z303 w/1.7 rockers) 1 point more compression. It does fine on just the center 250 cfm carb (3x2 setup) around town and on the highway.