StableLoads - Upper or Lower?

The 350 already sits tail high when unloaded, so I think you are looking at how to keep it level when weighed down. If you look at your upper overload spring, there is over 3" of sag (spacing) before it will contact your overload spring and engage it. Using upper StableLoad blocks will put it within 1" of your overload spring and allow it engage before the truck is pointing the headlights in the sky. I would suggest starting with this modification. Since the block does not touch the overload when unloaded, normal ride is close to the same unless you are rough roads where axle travel may occasionally cause contact (this is counter to Torklift's recommendation). If you find that you still need more leveling, then add the lower StableLoad wedges. These will adversely affect unloaded ride comfort, so it is nice that they can be swung out of the way when not needed.

I run both upper and lower StableLoads on my truck and do not need the airbags unless I am towing. By only pretensioning the OEM overloads with blocks and wedges, I can carry the TC with minimal sag.