Both ways work
Going to install mine tomorrow and want to make sure I understand. I remove the overload and the aal goes on the bottom of the pack where the overload was? The aal is around 2ft long and says for 1.5-2in of height.
If you leave the overload spring in, you will need to get longer U-bolts for the axle. Some folks recommend replacing the u-bolts anyway.
On both my '88 toyota and 2013 Nissan, I removed the overload spring (which is very heavy) and discarded it and put the add-a-leaf underneath. It raised the back by about 2" and the truck rides better and doesn't sag under load.
With the overload spring and the add-a-leaf, it would raise it about 1" higher.
On my old Toyota, it leveled out the truck. Those old 4 x4s used a torsion bar front end, and torsion bars work-harden over time, which is why some Beatles and old Chryslers go nose-high over time.
Leaf springs, on the other hand, sag over time. So when you see a 1980's four-runner or toyo pickup, they always look dorky nose-high.
The overload spring is an interesting animal. At least on my truck, it was hitting the bump stop when overloaded. I guess it prevents the leaf spring from inverting under heavy load. But it is so stiff and heavy, it really doesn't act as a "spring" as such, as it is only in contact with the leaf pack when you severely overload the truck.
5,000 miles on the add-a-leaf and I am very happy. Tows much better.