rear coilover lengths

[usa-69z]

I have been researching the lowering of the ridetech g bar rear end using shorter coil springs .I have found a couple of threads where guys are using 8" springs and 200lbs rates.. Is that too short to get the rear of my 69 down? Ridetech have said they use a 12" spring length with 200lbs rate.

so maybe a 10inch spring length? Has anyone achieved the desired result doing it this way? My car is street driven only.
I know this is not the best way to make this happen but it beats cutting the floor or changing 4 links.

thanx, Gav