Are the chainstays dimpled on your ST? I'm not sure of the year when that change was made - I believe it was 1988 or 89. My 1989 ST has dimpled chainstays with caliper brakes not cantis. I switched to 700c wheels mainly for the availability of better tire choices than 27" wheels allowed. I'm running Compass Bon Jon Pass 700x35 tires front and back with plenty of clearance. I also switched to dual pivot Tektro R539 nutted calipers and TRP RRL levers and the braking is very good.
After last year's Hilly Hundred, I decided that I needed a rain bike, so I went with the Selle Anatomica R2 saddle and I'm planning to add fenders. They will likely be a tight fit with the 35mm tires but I'm keeping the tires even if that means no fenders - those tires are that good. I have several bikes with the SA saddles and they are very comfortable for me on long rides. Your butt mileage may vary.
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Dale, NL4T