Kind of let this fall wayward, but got going on it again this last week.
One holdup was that I wasn't sure how I was going to make the external Durso overflow and be able to get the tank nice and close to the wall behind. Bulkheads are large and I want it to be modular in case I ever need to take it apart.
So I figured out how much threading I needed on the bulkhead and hacksawed off the excess. Sanded out the hacksaw marks with some medium grit sandpaper. Then, because the bulkhead was now so short, there was not enough room to have a threaded 1" close nipple for the 90 degree elbow AND for the "T", so I cut a close nipple down to size, threaded it in just enough that there's enough clearance leftover for the T to thread on fully, and then welded the two with some Weldon-16. For the T, I wrapped teflon around the remaining nipple about 6 or 7 wraps and got the T on. Smeared a little silicone around the edge for good measure. What I'm left with is a external Durso and low-profile enough that I should be able to get the tank to within 4" of the wall in behind.
The Durso here is 1" with a 1" bulkhead, minus some nominal radius for the close nipples holding everything together on the inside.
For the air valve I used a bushing to threaded 1/2", a close nipple, a 1/2" ball valve, another close nipple, into another 1/2" bushing back up to 1", then into a 1" coupler. On top of the coupler just sits a 1-1/4" cap. It just sits loose and has been drilled out on the top to let air pass. The open part of the coupler inside has a little bit of filter floss to act as an air muffler to keep the thing silent (hopefully). It has not yet been water tested so I don't know how gurgly it sounds yet.