Look at the ends of the torsion bar and you will see the three adjustment notches in the brackets under the package tray. Had the same problem after installing the go-wing on my son's Dart Sport. No problem holding it up after making the adjustment.
We found that the best way to do it is have someone get inside the trunk (with a shop light) and close the deck lid. Have someone on the outside standing by to open the lid if it latches (or keep a screwdriver handy inside). Use a pry bar to pull the end of the torsion bar out of the bracket and leverage it over to the desired notch. At the time, we were not concerned with scratching paint, because it is a black interior and easy to touch up. I guess you might be able to attach a ratcheting tie-down strap between the two ends to pull them towards each other enough to make the adjustment (never tried that).
Let us know how you do it,
Jerry