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?

Still no love on this topic. But for the record, I swapped in new JLM657 stock replacement coils and the rear sag is cured!. And they were only $90/pr from some Ebay no name. Which got me a baseline to spec custom coils off of.

Measured specs of stock:
5/8 dia wire
6 coils
free height 10.5 inches
flat ground both ends

Won’t have spring rate till I get them tested.

I installed a pair from an XJ8 along with the unibrace replacing the dog bones. No problem.