The 600cfm carb is plenty big enough but if it runs good without bogging at lower speeds I'd leave it. Through 60 years of messing with these old rides the most common performance killer has been "too big". Too big a carb, too big a cam, too big of exhaust, and too big of tires. A 307 at 5000rpm uses about 450cfm of air. There are tables at various carb sites but a handy formula is an engine turning 5000rpm (in this example) is doing 2500 intake strokes (the other 2500 are power strokes) so it's 2500 X 307 equals 767,500 cubic INCHES of air per minute which you divide by 1728 to give cubic feet which in this case is 444cfm.
