I did this one a few years ago, but never posted it here. I found a set of Shimano 105 STI levers at an antique mall for $5, but before I could decide which bike to put them on, my son asked me if I could mount them on his bike. He had taken a nasty spill when he drifted off the edge of the pavement while reaching down to shift, so he was apprehensive about taking his hands off the bars.
This bike could be a candidate for a Clunker Challenge, long term edition. Get a bike on the road for $100 and keep it for ten years, while putting at least 1000 miles on it. I got it around 2005 for $20. It was rough from sitting outside, but once I freed the seat post, it only needed new tires, bar wrap, and seat to be ridable. Even with the $20 pair of 8 speed wheels to go with the STI, I still don't have $100 in this bike, and it has accumulated a bunch of miles, both by me and by my son after I passed it on.
1989 Club Fuji with STI 105