Once the tank was apart, I sat the halves flat on the work bench, then elevated the narrow end 1". I then used a 1 1/2" thick wood block with a silver metal marking pencil laid flat on top, to mark a true line all the way around the tank half. I used a 4 1/2" angle grinder fitted with a cut-off wheel to carefully cut the line out; I then used my angle die grinder with a rolock disc to clean up the cut and true the edge to fit flat on my work bench.
Picture shows one side cut and the other marked....................