Rough numbers for the 351W, 450 to 650, just depends on what you are going to do with it, and how the engine is built. The 429 will take a range too, just depends on what you want to do and how the engine is built.
'89 XLT, 351W with some internal mods, C6, Ford Racing MAF
'66 Ranchero, 289 2V, with bucket seats, no mods at all, C4
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On the 429 I wouldn't go below 600 CFM, (it does depend on what your engine configuration is like, stock, mild, hot, wild). For a 351 at least 450 CFM. You will find that a lot of times you have to round up or down because they don't usually make the exact number you would like/need.
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