There is a simple calculation to work out the required cfm. Simply: cubes x max rpm divided by 3456. So we know yours is a 350 ci so say for your max rpm is 6000, 350 x 6000 = 2100000 divided by 3456 = 607 minimum so in this case I would go for the 650 cfm. If you want to build higher revs say 7500, 350 x 7500 = 2625000 divided by 3456 = 760 so you woud just get away with a 750. So as you can see it is all dependant on revs. The same equasion applies even on a blown motor, I have a 400 sbc running a 6-71 blower @ 6500 revs, so 400 x 6500 divided by 3456 = 752, so we run a 750 cfm..
Hope this helps