factory q-jets are rated in the 750-795 cfm range. & yes they work just on smaller engines like 305s & 350s. They work fine because of there spread bore design making them more forging because of the very small primarys.
I assume with the combo your talking about, this engine well be kept within about 5500 RPMs ( partially based on the heads your using).
With that in mind ,Id run no more than a 600 cfm carb.
A 750 cfm carb will run just fine on that 350 with proper tuning, but overall response will be better with a 600 cfm carb.