CAUTION: Well tanks are under pressure even after being disconnected to the water supply. Make sure you deplete the pressurized air in the tank at the needle valve (usually on the top of the tank) before drilling into the tank.
I drilled a 1/4 hole right below the middle seam in the tank (where it was welded together) Once I had the hole, I took a jig saw and followed the seam all the way around the tank cutting it in half (pretty easy using a metal cutting blade).
Once it was cut in 1/2 I removed most of the rubber diaphragm using a box knife. Add wood, a little lighter fluid and enjoy! I plan on repainting the outside of the tank with high temperature paint once the old blue paint is burned off. (and until the rest of the diaphragm is burned off I wouldn't recommend roasting marshmallows yet).