Hi and welcome to the forum
Lowering a 306 is not quite as easy as just replacing four springs. It is at the front and a variety of suitable springs should be available from the usual sources but at the back, it is a case of removing the trailing arms and replacing them on the torsion bars one or two splines around to achieve a lower ride height.
This is easy in principle and is a doddle on a brand new car but over the years, corrosion sets in around the rear beam and the whole job becomes a living nightmare. As an ex-205GTi owner, I know
The job on a 306 is precisely the same as on the 205GTi. Have a look at the
205 GTi Drivers website and on there you will find detailed instructions in the Articles section on how to both lower the rear and if necessary, how to refurbish your rear beam as I expect when you pull it apart, some remedial attention will be needed
Hope that's helped a little...
