Just and FYI the 240 front springs changed in 90-91 from the softer 13.4mm to the 13.8/9 mm on all fitments, wagon or sedan, the same as early turbo's.
The 14.2 mm are in fact the GT size and available from B&G/Leijsfors of Germany. I have had a set of the GT springs on my '91 245 for 7 yrs and 150k miles, very good spring, probably teh same as sold by ScanTech.

If you put in stiffer front springs be forewarned that if you leave the rear stock, especially the earlier stock size, and use stock shocks you will be setting up the car for a roll-over, as the rearward pitch of weight into a corner will cause the back to dive and front will not - flip!
If you change the front's to GT, at least put in overloads, later wagon springs, and/or high pressure gas shocks
It is far better and safer to add stiffness to the rear springs with gas shocks, and keep the front struts ydraulic and a modest increase in spring stiffness to keep a good ride. Sitfining the front springs doens't help with load carrying ability very much.
Oh and about rear axle centering, if you put the GT/Overlaods on with Bilstein HD shocks and struts like I have, the ride height increased 1/2" to about 11 1/2" overall. The small increase in ride height is still enough to re-center the rear axle with the adjustable panhard bar. I have the IPD panhard bar, and although the adjustment amount was small, like 3/32", the biggest difference it makes is the much bigger poly bushings on the adj bar keeps the rear end planted in corners much better than the stock panhard bar, FWIW, a better set-up.