Need rear coil spring specs

I’m going to get some custom rear coils twisted up for my 93 Vanden Plas 4.0 (serial # 671xxx) but I need to know the starting point of stock springs.
So far all I’ve found is:
free length: 26.7 cm
dia: 13.2 cm (but is that ID or OD?)
approx 10 coils, flat cut both ends

What I need is:
Rate lbs/inch
Installed height
Compressed height

Any help?