It'd probably by tig welded. I want it to be lightweight.
I'd also probably have three water bottle bosses (the third on the underside of the down tube), as I am into long-distance riding.
Won't the chainline spacing have most to do with the bb spindle length? The bb shell will be 68 mm wide. I'd probably go with a 130 dropout spacing. How does dropout spacing affect chainline? I wouldn't ever use this bike fixed, so I'd probably end up going with a single speed adapter on a gear-intended freehub body, which will allow me to adjust the chainline.