For the Touring spec setup of 19" front, 18" rear, I have 100/90 on the front, and 110/90 on the rear. The chain guard indentation in the rear mudguard, I have to bend it out, just below the RFM pivot, to keep the tire from rubbing.
I believe 100/90 is the closest modern equivalent to the original 350 tire width designation, while 120/90 is the closest modern equivalent to the old 400 tire width. But I haven't dared try to fit a 120/90 to either of my Vincents, one with 19 rear rim, one with 18 rear.
I believe 90/90 is close to the old 325 tire width, while 80/90 would be closest to the 300 width.