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There is a bolt near rear of torsion bar (the end closer to rear bumper) which will increase or decrease the tension on torsion bar when you adjust bolt.
as the rear is torsion it is not straight forward. the torsion bars have to be removed one side at a time, with the torsion bar out simply adjust hub the required amount and replace torsion bar. then do the other side. this can be hard as the bars can be difficult to remove. best left to professional should cost between £60-£100
Under vehicle by end of torsion bar (side closer to rear of vehicle) there should be a 6-point bolt which when tightened or loosened will apply more or less tension to torsion bar.
on the rear cross member where the torsion bars mount there is an adjustment screw
Take the truck to your local Chevy dealer and have them adjust the torsion bars to lift the front and then do a realignment. About 100 bucks. New torsion bar keys are not needed.
The 1964 GMC does not use torsion bars, in 4x4 applications it uses a leaf spring in the front. The 2wd uses coil springs. Both models use leaf springs in the rear, the Chevy version used what is called a truck arm suspension with coils in the rear.
You remove the metal retaining clips on the back side of the frame / cross member that stop the torsion bar from sliding out. Figure out a way to clamp something to the torsion bar and smack it with a hammer driving the torsion bar towards the rear of the car. PS they do sell tools made to clamp on to the torsion bar. I would also suggest having the wheels off the ground and hanging. Also loosen the nut that adjust the ride height.
Front is torsion bar and rear is leaf spring. Both front and rear have anti-sway bars http://en.wikipedia.org/wiki/RPO_ZR2
For the front end you can adjust the torsion bars by tightening or loosening the torsion bar key adjustment screws. It should be an 18mm hex head bolt. There should be a few inches of play overall; any further adjustment would require changing out the spindles or lowering blocks. l Since the rear end is traditional leaf springs there are a lot of accessories you can order to raise or lower it.
If you look at the end of the bars towards the rear you will see a crossmember that thay are in . look at the crossmember and you will see a bolt head that is in a bracket that is right beside the end of the bars, that is the adjustment bolt for the torsions bars.
Revised leaf springs in the rear. Revised torsion bars and torsion bar cams (a.k.a. 'pork chops') in the front
On the rear of the bars, there is a bolt on each side coming down through the cross member. Tightening these two bolts will lift the front end.