I run dual Carter W-1's on a stock 250 inline six. It was the best move I made as far as drivability, and mileage. Performance gains, maybe a little, not enough to cover the costs, but the mileage increase more then covers it. Six cylinders have terrible fuel distribution, outer cylinder run lean, inners run rich, theres no way around it with stock equipment. I have messed around with carburetors for 20 + years on my truck, and by far the Carters work best. They have plenty of adjustability and are small enough to keep air velocity high. My intake is home made with small runner diameter, this helps keep air speed up but probably hurts high rpm running which I don't do much of anyway. If you watch e-bay, you can pickup metering rods for cheap, and there are a lot different ones available. My set runs stock jets with different metering rods, the rest of the carburetor uses all stock replacement parts. Do yourself a favor and get an Air/Fuel ratio gauge, it will cut down on a lot of guess work.
