When doing the research I discovered the cast LS weighs less than the 258 I-6. The aluminum LS is about 100 lb under that.
I got my 5.3 from a running suburban. Bought the whole vehicle, then pulled the engine/harness/pedal/ecu/etc.. from it. The remainder was hauled off to a metal scrap yard. If you spend the time, you could probably get an engine for free by parting out the donor vehicle, but I had no desire to commit that much time to the process.
My 6.2 came from a wrecking yard and is out of an Escalade and needs an overhaul. (Benefit to the 6.2 is weight loss and the added low end torque for slow trail riding)
With a lifted vehicle, the engine is not really down low. Any added weight on the axles would help the center of gravity, the engine would probably hurt it some. But too much weight on the axles hurts the suspension as it must be controlled by the springs/shocks. It is a balancing act, and I personally feel any weight lost is good weight lost.
Gen 3 is easier to deal with, less complicated. Gen 4 you will generally need to modify it to be like a gen 3. (displacement on demand disable, variable timing, etc...)