Gerst Dominator Series 4 Link
I think you could section a stock gas tank for clearance if necessary. I couldn't see losing that much capacity.
Sectioning a fuel tank is a colossal pain in the butt. The original tanks are dual layer, so, that's pretty much a non-starter if you really have an original tank. If you have an aftermarket single wall tank it has to be boiled out at a radiator shop to allow welding on it if it's had fuel in it. Yes, you can redneck it and hope it doesn't blow up, but that's your problem because I'm not welding on anything that used to hold fuel unless its been properly treated. Then you have to lay down a perfect, fuel tight weld. Then you should have it pressure tested to make sure you're actually right about that weld before you fill the tank with fuel. Since water has larger molecules than fuel, even pressure testing with water
might not reveal all of your possible leaks. And when you're all done, you've screwed up the zinc coating they put on the tanks to keep them from rusting, so, you either have the tank re-treated or sealed so the weld doesn't rust. Most gasolines contain alcohol and are oxygenated, so the alcohol sucks up moisture and the untreated metal will rust in no time flat.
Oh, and and zinc coating has to be stripped back from the area of the weld on both sides prior to welding, you do not want to breathe zinc fumes. Search "metal fume fever" if you don't believe me.
Honestly, it would probably be easier to fabricate your own fuel tank than section an old one.