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IMHO,
Both the 1850 and the 4220 (600 and 660 cfm holley carbs) are viable alternatives. As are the dual 390 or 450 cfm units. Your 427, 428, 454 et al will never pull more than 900 cfm. That said I really think for day to day simplicity the 1850 vacuum secondary is the easiest to live with. For full disclosure I run the 660 cfm carbs for max power and 450's for cruising.
I really love my 660 cfm Holley mechanical secondary carbs but they are a compromise, all carbs are. What works well at 2500-3500 rpm is often different than what will work at 8000 rpm.
If you are fouling plugs I suggest pulling a vacuum test. Your power valves shouls be at least 3" of vacuum less than your vacuum at idle, That is if your engine idles at 12 your power valves should be a 8.5 (standard valve for a holley), but if you get 5.5" of vacuum at idle you might want to use a 2.5 or 3 power valve.
A good Holley carb book will help a lot, good luck!
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michael
A man that is young in years, may be old in hours, if he have lost no time. But that happeneth rarely. Generally, youth is like the first cogitations, not so wise as the second. For there is a youth in thoughts, as well as in ages... Sir Francis Bacon (1561-1626)
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