A 4 1/2" angle grinder with a thin wheel is probably the easiest BUT MOST DANGEROUS way to make the initial cuts. A drill or hole saw is a slightly better choice since it leaves a rounded corner, reducing the chance of a stress crack or tear. But an air hammer with a slitting chisel is safest & quickest.
If using a wheel, remember that it sprays red-hot steel either up into the truck, or down onto the fuel tank. Since this truck had no camper shell or interior, I let it fly up & wore safety goggles & ear plugs.
