Use a cutoff saw.Or you can use a regualer grinder and grind the head off.
Cutoff saws are quicker but either one is a pretty basic approach to solving the issue.
Try to keep in mind where the fuel tank is and keep sparks from flying in that direction, if possible.If needed, wet some rags and pack them in the work area around the tank, so sparks dont get near the tank.Cant be to careful around gas, with stray fumes and all. Keep a fire exstinguisher handy.
Not trying to scare the hell out of you, just trying to keep you safe, and if necessary ...prepared.
One alternative would be to heat the bolt with an inductive heater, if you can rent one as buying one will set you back 6-700 bucks.
Fact is stranger than Fiction