Like I was telling my buddy a few weekends ago after I got his new motor's timing, carb and transmission kickdown all dialed in and running way better than it was before all the basics were set up properly, they're called speed secrets for a reason. Not everyone knows them.
I can count on one hand the number of cars with properly set-up slapper bars out of the hundreds cars I have seen in my life that had them bolted on. You are right, they will wrap the front spring segment if the snubber is placed in back of the front spring eyelet. Also, they won't do anything for you if the snubber is too far away from touching the spring eyelet.
Most slapper bars do not fit right straight out of the box. They typically have to have the rear mounting perch cut, adjusted and rewelded to properly place the snubber (of which you should cut about 1/3 of it off) directly over and just barely touching the front spring eye right where the front bolt attaches through the leaf spring.
Once properly adjusted, make sure to use quality hardware to attach it firmly to the spring, retorque them a few times and check them regularly.
When properly set-up, the work exceptionally well on the semi-elliptical rear leaf design of Mopars.