I also drilled the last hole in the tank. It took about 2 hours. The challenge this time wasn't clamping, it was dealing with the height of the steel tube. It was a tight fit. In the picture below the Quill (the part that holds the cutting tool) is all the way up and I've only got an inch or so of clearance.
The angle iron wouldn't fit into the jaws in a 'normal' configuration, but this made the jaws have a much deeper throat. This way they could hold the angle. Finally a picture of your intrepid machinist:
* Timesheet updated.