Note that the Sora shifter operates a little differently than your 600 STI shifter. The Sora shifter has a thumb-actuated release lever that's mounted on the top front portion of the hood and is difficult if not impossible to reach from the drops. I find it impossible to upshift from the drops with Sora. If you're okay with that then go ahead. Another option as someone has mentioned is to use Campy 9-speed shifters or hop on eBay and seem if someone is selling NOS 8-speed STI shifters.