Well I would get 12" half blocks and stack them up on the sides, again wall tieing the piss out of them, every course.
Lay a beam course upside down on the double angles.
You would be wise to put some insulation over that metal mess you have there and make sure it is sealed off nice. I have some reservations about pouring the cells with re rod, but you might want to grout them at least going over the lintel.
I would have to see it in person, but i might build the rest of it after the angle irons out of just red solid bricks. As it tapers in you will want to fill it with some intelligence in that 2" gap. If you build it with 8" block you will be left with just filling it with rubble and mud. That wont be good enough on that old metal. It should be supported and tied in somehow.