First, GM used Qjets on engines from 230 cu in to 500 cu in. The basic carb is the same, and even the ones on the small motors flowed 750 cfm. Check the Rochester carb data sheets to find the jets and metering rods used on one of those smaller motors that's close to the displacement of your six. That will get you in the ballpark. And flooding has nothing to do with engine size or carb jetting, as noted it's a needle and seat or float issue unrelated to engine displacement.