I have not done this, but my feeling is if you install springs from a 325, it may give the 318 an unwelcome "nose in the air" attitude. Just cutting some length off the spring will change the spring rate, possibly making the height good again but being way too stiff. So, if you are going to spend the money for performance parts, put the right ones in and get value for your jack, instead of spending to make matters worse.
Geoff