I have the same problem. There are a couple of notches remaining on the spring bracket and I wonder if I can pick up enough tension if I stretch it to the longest position notch.
The Haynes manual, page 198, lower left corner, Figure 10.17, shows a spring being stretched. It shows a heavy wire loop around the spring end hook - picture an oval wire loop. A horizontal lever bar is then passed through this loop and the end of the lever is placed behind the shock bushing. The handle end of the lever is toward the center of the car and the spring is stretched by pulling the lever toward the front of the car.
I haven't tried this yet. Looks like a lot of spring force involved. Don't know how it compares to the rear lid torsion bars for body alteration power! I have played with those before!
Let us know how it comes out!
Mike