I think the 350 twin and 350/400F share the same wheels.
I'm waiting on some spokes to lace a WM2 (1,85") front and a WM3 (2.15") rear. Standard tires for those are 90/90/18 front and a 110/90/18 rear. You can squeeze a 100 front and 120 rear on there if you wanted to.
If you wanted to go up a notch, do a WM3 front and WM4 (2.5") rear.
I don't think anybody would notice the spoke count, especially with dual disks in front.