I haven't done it yet but have trial fitted - I have taken the inner and outer part of the arches apart as I think that's how they must have single side spot welded the outer arch all the way up the vertical flange to the B Post at the factory.
It's a tight fit but it seems just possible to plug weld in the outer (I don't have a single sided spot welder!) and then fit the inner "inside" the outer and spot / mig weld the inner and outer together in all th factory places.
Clamping everything up nice and tight is pretty important to me so that all the seams are nice and narrow and consistent. I am going to use a combination of "clecos" for the b post seams and some long reach / deep throat (ooer!!) g clamps for the joint with the arch tub.