Holley 500 CFM 2 Barrel #0-4412S on a Slant 6

If you read the Holley book on carbs, they emphasize the biggest mistake is too big a carb. For the low-end, a 4-barrel of the same cfm rating is much better than a 2-barrel, especially if small primaries (ex. spread-bore). They also promote vacuum secondaries (i.e. Holley instead of Carter AFB/Edelbrock), so the secondaries open as needed not when an inexperienced driver floors it.

However, you must fit something to your 2-barrel intake. There are 4 to 2 adapters, if you have the height. I have always heard a 350 cfm 4-barrel is perfect for a 6-cylinder (avail, but rare). 500 cfm sounds too much, especially in a 2-barrel, unless you have a turbo or such. Consider the 2-barrel Carter type which came on my '65 273 and most 318's. I bought a new one for $25 recently. Don't know the cfm rating, but certainly much less than 500 cfm.