To adjust slapper type bars, there are really only two ways to do it. There are aluminum "wedges" made that go between the spring pad mount of the traction bar and the spring to change the angle, or use longer/shorter snubbers. Some snubbers can be cut to shorten them.

Do slapper bars work?

They work great and don't effect the ride, however, a lot of slapper bars (cheap ones) were too short and the snubber didn't contact the spring eye. Insteady, they would contact the spring and if the launch was violent enough, bend the spring. So true!

How do you adjust traction bars?

You should adjust them by pushing the axle back, I.e. lengthening the bars. If you lift the rear and let the suspension droop before you preload you will be able to put more pressure on them when the truck is back on the ground, but it will stiffen the ride.