There's a few things here to note:
1. The Torsion bars are a spring and they act like a Coil spring and also set the static ride height
2. If a rear damper and spring is being used the two spring rates need adding together (EG 160Lb Coil + 130Lbs T-Bar =280lbs)
This is the total spring rate, but the actual wheel spring rate due to the (.55 Motion ratio) is in this example 154lbs
Hence why RobWright only fitted the Damper and left the coils out, making the rear rate and ride Quality far better.
When the desired front spring platform and ride height has been set, the rear torsion bars will need to be pulled out and
put back in to produce a lower ride height, they are located by a spline in the centre of the torsion beam and another spline in the spring plate which is bolted to the trailing arm,
So wherever they sit their spring rates and it's resistance will always act and be additional should you also be using Coil springs wrapped around the damper (So Gerry your rear spring rate will be your S2 T-Bars + the Coils , from the get go... there's no way of delaying them to come in as they are a permanently fixed spring.
(As Waylander suggests, setting the front ride height too low and the wishbones will not only put more stress on the vulnerable ball joints but also the car will roll more as the Roll centre has been changed adversely)
Re-indexing and setting the rear ride height is not an easy task, especially with bolts etc..being 30+ years old!
I'd recommend taking them out, rather than re-indexing them which will then enable you to adjust the ride height far easier with the coil over dampers, and should you want to go stiffer the rear coils are again far easier to change.
R