Here is the tank I used from Dax.
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Depending on the geometry of the build and whether you want the tank above or below the rack, you may need to lift the rack so that the tank is above the carb. Here pics of my finished build. http://motorbicycling.com/f15/first-build-diamondback-outlook-10802.html
You could also mount the stock tank on a rack. That's what I'm doing for a build for my father in law just to give more clearance to get his leg over the top bar.