The two long bolts at the rear are the strap bolts and they shouldn't turn, they are the end of each strap. The remaining shorter bolts are welded inside the crossmember. If the weld has broken you're best off cutting them.
I'd lower the tank, by far the easiest and least invasive for removal.
While you have the tank out, look inside the vent tube opening and you'll see a hose inside the neck. Remove that tube from the neck and your tank capacity will go from 15gal to 20gal.