Looks good so far. Do You plan to make "doors" for the ends? Closing it in somewhat will make a huge improvement in performance. A popular and effective solution is a 2" square hole in the back door at the bottom to pass long stock through, and a front door that ends about 1 1/2" up from the forge floor. For a lot of work, these doors can remain closed, that ammount of opening is plenty for the combustion gasses to escape, and allows pretty good access.
If You try forge welding, keep the flux off the refractory.
If You close the ends in with insulation, You can probably run it from a 20# cylinder fairly well , if it freezes the tank, use a larger one.
Do You have a kiln tile or thin firebrick covering the floor? They keep the wear & tear off the refractory board.
There is a reflective lining called ITC 100 that improves efficiency, anvilefire.com sells it. Larry Zoller sells Plistix, a cheaper but somewhat less effective alternative.
http://www.zoellerforge.com/index.htmlWhen it comes to tongs, the "Off Center" brand are the best, Tom Tongs are cheaper & pretty good. It is hard to make tongs this good Yourself, but it can be done.
If You find Yopu are doing mostly smaller work, You can decrease the interior volume with more insulation and cut back on fuel use.
For anybody building a forge, here are a bunch of pictures of various ones, hosted on a smithing forum.
http://www.farwestforge.com/Forum/bsgview.php?cat=F