Raising Rear End with a Lift Block kit on my Slant six
Yup. those will lower the car.
Either find a "spring shop" and have your stockers rearched
or find a junker and rebuild the spring stack, adding a leaf or two
Gabriel/ Monroe / others? air shocks
Air bags
ANYTHING is better than those dammed rear shackles, here's the main thing wrong with 'em:
Other than they look totally silly, they allow the springs to move from side to side, and can actually allow the rear axle to "steer" somewhat. That is, you get 'er leaned over in a curve, the shackles are forced off to one side, the rear axle is allowed to "slide" or tilt or whatever word you want.
In other words it completely destroys all handling.
Speaking of handling, going around wanking up the rear without taking a look at the front suspension, isn't gonna do the handing any good, either.