First let me clarify the language - the "strut" is the entire suspension unit. The wheel and hub bolt on at the bottom and the "strut-top bearing" bolts into the "Strut tower" at the top. The bit inside that moves up and down is your "strut insert" or "damper", or in laymans terms "shock absorber". When off the car with the strut top bearing, spring and damper removed - the bit you have left (that the wheel bolts to) is the strut.
I thought all of the 325's were the same. There are two strut sizes - 51mm and 48mm. The 48mm was common on 320i's and the 51mm was the one for the 325's.
The collar that holds the damper in the strut is specific to the damper too. You buy them from the damper manufacturer. ie Bilstein make collars for their dampers. So, you may be able to fit the dampers in a different strut if you have the correct collar, or you may not depending on whether the damper was designed to fit different struts.
What you will find is, that the entire strut assemblies are interchangeable. So why not fit the springs you want with your bilsteins to the struts the bilsteins fit in, and mount the whole strut on the car you want, and put the rest on the other car?