Have got trusses or hand framed rafters? Cut a hole at the end of the wall to run a 2x4 down between floor joists and block it solidly to hold your wall stiff.
I am still concerned about your roof, Evan if you have trusses you want to have a close look at those, because with newer houses, roofs change directions and some trusses are tri bearing. When or if you have something like this you will find 2 or 3 studs nailed together on the main floor and in the basement. Just make sure before you cut.