I know, I know, its different but, anyway... This morning, I put an old 5 speed freewheel Mavic wheel in place of my 9 spd Ultegra Elite rear wheel. I had to add some washers to get to 130 mm width instead of the 120 mm of the old wheel. Then, I tried the bike on a trainer. I could get to some cogs ok but 3 out of 5 could not be reached with the Ultegra shifters. Those are indexed for narrower cogs and spaces between them. If the Sora brifters are 7-8 speeds, I think you will have the same problems I encountered. And if you try adjusting the rd, it will eventually work for some cogs but not for others.
Others can confirm (?) if the STI 6 spd downtube shifter is indexed for the same width as the Sora ?