OK! Now I have time to put my thoughts into words, pictures, and wild gesticulations.
My first thought went to this thread:
http://forums.sohc4.net/index.php?topic=43832.15Then I got home and whipped out my photoshop and did a 3-pic, step-by-step visualization of what that'd look like on my bike. Your results may vary.

Here, I just copied the tank and tilted it to fit the angle of the bottom of the seat. I'm figuring in real life application here, you'd want to weld on tabs to bolt on roughly where the seat bolts on back there, some sort of bracket. I left it mostly transparent so you can see overlap.

The blue marks are just me thinking about where it could/should be cut, but I'm just thinking to keep it in line and maintain the shape you want.

Here I did hack and slash along the line, did a lousy photoshop job of removing the grab bar, and pasted on the nearby truck's taillight. The only point of this being a rough mockup of the shapes that could be achieved while maintaining a stock stepped seat (though very few of us have 650s...)...
hth?