So I am resorting to techniques I have not used since I was a kid building go-carts to get this hole made for the chimney. The hole saw worked OK for getting through the actual side wall of the tank but where it is cutting vertically through the end of the tank the wall thickness is at least 1/2". It is easy enough to drill this stuff but it absolutely kills blades of any type. So getting back to the go-cart thing I am drilling small holes around the perimeter and when I get the piece knocked out I will clean it up with a file or die grinder.
Nothing wrong with that! Now you're talking my metal working skill level!