Have you tried a Norton tank? And when you say norton are meaning one of the featherbed frames have on them? If that is what you are looking for try to do a google search for "Triton motorcycle" Tons of Brits make that kind of stuff but it is slim in the states. Most everyone here wants a CHOPPER, that is what's cool right now. Just kidding I like those also but I know (just don't have the link) of a guy in England that custom builds that tuff but you better be ready to drop the moola to get it.
What other works already done? I only ask to get you to think about a 'end plan'. A tank can change can be dramatic, makeing yours slender and longer would be cool. But you have hundreds of options if your plan is make a lot of other changes. For example, those bikes look great with a little rake, lowered, solo seat, flat fender, 3.5 FatBobs and drag bars.