Sidepull and Canti/center pull should both be utilizing short pull brake levers. I would just get a brand new set of short pull levers and then you are good. They are pretty cheap. If they are mistmatched at all you will just have weak or extremely strong braking. You will still have braking power either way, just test it somewhere safe before you go on a ride so you know what to expect. I have some bikes with those old dia compe side pulls and they feel about as weak (due to the technology of the brake, lever, and braking surface) as using a long pull lever on a short pull caliper but it still stops.