Here's a few pics of a Sportster gas tank that I modified for my friend Rich. I cut it in half and removed about 1.5" from the center, put a flat bottom on it with a high mounted Frisco style tunnel, made custom mounts, and relocated the petcock and filler bungs.
We can do this with your stock Sportster tank for around $275. We're also capable of doing many other mods to any gas tank. We do everything from narrowing and Frico'ing to just relocating bungs or installing mounts.