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 new body style 1500 trucks there is no torsion bar. there is now a coil over suspension. to raise it and level the truck you will need a "leveling kit" typically comprised of a metal spacer...
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...