Coilover springs can be fitted to front of the Datsun 1200. Or to the rear of sedans (but not coupes).
Ground Control has fully adjustable coilover struts, for $199 USD a pair. See stiffer suspension
Coilover kit from Ground Control
Budget Springs
Instead of buying new, 2" diameter expensive springs, re-use your existing ones by using BMW removable spring seats:
reference: removable spring seat