Ya the runners are made as two halves then welded together. The profile was turned on a lathe from 9" solid aluminum bar then cnc milled to shape to match the flange and plenum. I did it this way so I could keep the inside of them perfect with a nice radius in the corners and stuff.
I welded everything with it bolted to a head so it wasn't too bad. I did machine the flange true after all welding was done though. I just seal it to the head with permatex right stuff gasket maker.