I once again outsmarted myself... attaching the chainstays first, due to the super steep chainstay and DT angles, meant that (doh) I had to slightly miter the downtube to the chainstays. Not your normal procedure, but a minute or two with a small file took care of it. Once again, welding the tube in was...weird, since I'm used to having the seat tube to work around (not to mention working with a full front triangle, rather than just this "spine", which I ended up welding in the fixture, rather than on the table).
Next up, the seat tube (a funky job) and toptube(s). This is one of the more involved and odd builds I've done in a while, but I think it's going to be very neat when it's done.
We decided on a 44mm standard head tube on the off chance that a tapered steerer appropriate for a rigid fork becomes available someday, but initially at least, the bike will run an inset 1 1/8" headset and a standard steerer fork.