[Chaplin]

A 750 vac secondary will work on a 350, and you could also use a 900, but to what benefit? As noted, a vacuum secondary will only take as much fuel as the vacuum draws- and even assuming 100% volumetric efficiency, you can't use all 750 cfm. And unless you're drag racing, I'm guessing most people aren't turning their motors to 7k on the street, so a 650 is more than enough- as explained in my previous post. Plus, with a 750 you will have a drop off on the bottom end- but, yes you can use a 750 or even a 900 with vacuum secondaries if you want.