I suspect if you attempt cutting where no seam is present, dual angle iron would help to cut straighter. I'm using the top half for a compost rig. It has a new (two piece, seam centered) sheet metal bottom and will have a shaft going through the center with a hub on each side. Turned-- horizontally, it will rotate like a squirrel cage.
The bottom is in, inside wall coated with tar cement and a plastic stuck over the tar. Next I'll cement a large wooden hub to the flat bottom and a sheet metal hub (12 ga.) to the conical top with a 1 inch galvanized pipe to work as a axle shaft. Turned horizontally and pushed up into a concrete carrier it will rotate and I'll fab up some paddles or blades to agitate the contents. (my, my, my. That didn't seem very primitive at all--- Y'all) I like to think, that I'm only brushing up against the iron age. At least I used old bolts with square heads and did the work barefooted. Wrong. I had my sandals on.