If you use a saw, make SURE you don't puncture the gas tank or cut the fuel lines & wires. Even if it doesn't catch fire, it's still expensive to replace. This body is on 3" lifts, and this blade would still hit the tank if I wasn't careful. A jigsaw with a short metal-cutting blade is a much better choice, but a slitting chisel in an air hammer is even easier and MUCH safer; both for the operator, and the truck.
A slitting chisel in an air hammer is a FAR-BETTER way to cut the hole, and doesn't require any prep.