So, you want to put the 105 on a 1987 520 that you're building up from the bare frame? Am I understanding this right?
If so, just get 700c wheels. I doubt there are that many good 27" wheelsets available with cassette hubs. If you find one, go for it. Feel free to use whatever wheel size you want. But if you are buying new wheels and tires I would just go with 700c.
Your only potential issue is getting the brake pads to drop 4mm, which has not been a problem for me yet and I seriously doubt it would be a problem with cantilever brakes. They are pretty adjustable.