I too had this same problem on my 1984 Trek 720. The canti posts had a spacing of around 60mm making it impossible to use 700c wheels with the stock brakes. I ended up getting the Shimano CX-50 catntis mentioned above, ditching the stock pads and spacers and replacing with normal road pads with the wheel centering tabs ground off. Works a treat. The rear was simpler because of the wider canti spacing, just used the stock pads and the thinnest spacers provided with the brakes. Works with both 27' and 700c no problems.