I have a propane tank build. The ends of the tank are spherical in shape. I will have a 4 inch round stack and will need to cut a hole in the end of the tank for this vertical stack. I can weld well enough but I'm not a metal worker and have question. Is there a simple way of determining the shape of the hole I will need to cut as it won't be a circle. I don't have a torch or plasma cutter. Can I cut a round/oval hole with a 4 inch cut off wheel in the thin stock used to make these tanks, then grind the rough spots?. Is it possible to find good blades for a jig saw or reciprocating saw/sawzall which will cut this steel?