I'm a bit confused on which length spring I need for my Marder. Just ordered a set of 20mm shocks and can't figure out which springs I need. I've found 50mm and 60mm for the fronts and 90mm and 100mm for the rears. Both say they fit the Marder but it leaves me confused. What's gonna be the difference between the two? I'd measure the ones I have but unsure if they have compressed any being almost 10 yrs old.