u can also get custom springs made up to any rate and size, try 4wd custom spring places etc
if you wanna test em out get soft springs say 7/5 and get the angle grinder out when u want harder springs
60 mm OD = 40 mm ID, I have never seen a spring that small.
If you meant 60 mm ID, then;
Spring rate = 290 lbs/inch (5 kg/mm)
350 lbs/inch (6.3 kg/mm)