Need to stiffen my rear 68 dart suspension

As for the Hotchkis lowering the rear, if you still have the original springs your rear end is probably already lowered that amount. Remember that while a spring's rate never changes, it never returns to exactly the same shape after each compression. That's why springs "settle" or sag over time.

If you are wanting to raise the rear for that "jacked up" 70's look, (some call it the stink bug look, I think it makes a car look as if it wished to be mounted from behind by an Oldsmobile) this will hurt handling. Lowering moderately is almost always better for handling.

As stated above, a rear sway bar will help a lot, but don't even consider it until you have a front bar. What are you running for torsion bars, an what is the general condition of your front suspension? That is, how long ago was it rebuilt?