As Fred says 4 bbl and 2 bbl carb cfm ratings cannot be compared directly as the the carbs are flow tested at different vacuum ratings.A two bbl of the same CFM rating as a 4 bbl will flow about 15-20% less than the 4bbl.I know of a few guys who tried a single progressive 2 bbl Pinto (Holly-Weber) on inline 6's.Part throttle power was noticably reduced by the small single primary throttle, while full throttle power was improved only slightly.A typical Holley style 350 cfm 2 bbl should work just fine on a 250 engine.I saw a Rochester 2GC as suggested by Olds 42 used with excellent results.You will be using an adapter which will restrict the flow into the intake anyway.Since your engine is stock maybe the easiest approach is to use another monojet or an older Rochester B.
