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.
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.