I have an '84 620. It's pretty much retired now, but I rode it a whole lot for 17 years and now it has a place of honor over the workbench. I loved that bike; riding it saw me thru a whole lot of growing up. I put 700's on it a few years in and had no problems using either the original DiaCompe cantis that it came with or the Mini-V's that came a little later. Have a look at how the cantis are sitting on the 27" rims now. If they're not near the bottom of their adjustment range then you shouldn't have any trouble making the switch to 700's. Or you could find somebody you know with a set of 700 wheels, and pop them onto your bike for a minute just to see how they'll fit.