It's really no big problem, as it'll be a 285 cfm carb most of the time.
In the old jetting post,
http://fordsix.com/forum/viewtopic.php?t=8106, there was a bunch of listings in line 11, 18 and 19 which showed primary,seconday and venturi sizes.
The 570 cfm is a new listing based on the old 4160 series 550 cfm carb used in the early 70's. It combines a 1.5" (38.1 mm) bore with a 1.1875" (30 mm) venturi, along with a better annular discharge boster venturi, which picks up another 20 cfm over the earlier truck type booster they used to run
Line 18|#4160-|550|4-bbl-|1.5000--|1.1875--|
The orginal 550 was a 4160 carb designed to fit in the gap between those who found the 465 cfm too restrictive and the 600 cfm too big. In the 231 to 300 cfm area, the only option was to make a smaller throttle bore 600.
That looses 50 cfm through a 62.5 thou (1.5 mm) smaller throttle bore, and 62.5 thou smaller primary and 125 thou (3mm) smaller secondary venturis
Old 600 listing had specs
Line 19|#4150/60|600|4-bbl-|1.5625--|1.25000--| On the secondary side, -|1.5625-|1.3125-|
Old 390 had listings
Line 11|#4150/60|390|4-bbl-|1.4375-|1.0625-|
The newer 570 is really a smaller 600 cfm carb. Cars with the Valiant style 245's used in the AMC's like carbs around this size. You'll rember the settings you list many moons ago that had a 600 carb on the 245 AMC engine.
In a light car, you'll get no bogging down low, and no problems with idle quality with a 570 cfm. The standard jet configuration on Holleys is often the total venuri diameter, divded by 20 for little carbs like the 390 to about 18 for a 600 to 850 carb.
In your case, it should come with 62's all round. That will give good power to about 225 hp with much better low end characteristics thanwith a 500 Holley, and much better part throttle economy.
Hope that helps.
