In my case, I used 1 1/2" black pipe (schedule 40) for the plumbing to feed air to the fire.  I found an elbow with a 3/4" nipple incorporated into the bend.  I hoped this would give me better air-flow and still allow me to easily dump any ashes that fall through the tuyere, one of these days I may actually get around to welding something to the plug to make it easier to open when everything is hot.

Setting the floor of the firepot in place

     The floor of the firepot is a place into the firebox by standing it up at one end, lining up the slats with the flange and lowering it into place with a door like motion.  The floor should fit with a bit of a gap at all edges to allow for expansion when hot.  The one pictured has about a 1/16" gap on all sides.  You really don't need much, but you do not want to pound the floor into place each time you assemble the forge.  It certainly won't last very long under such abuse.