Ford manual for '97 states that it is not necessary to replace springs in pairs, they can be replaced individually.

I think this is correct when replacing due to breakage. If it was a 10+ year old car and you were replacing due to sag then I would replace in pairs.

98 Mystique LS v6 atx 110,000km
97 Contour LX v6 atx 125,000km
stock CD on LS, cass on LX
spoiler, tinted windows on LS only
4 wheel discs on LX only
All other available options on both