Does good upper body strength reduce the stress of lifting objects (e.g. boxes) on your spine?

Lifting Objects such as Boxes can be thought of as,

Lifting Object Off the Ground
Carrying Object Lifted from Point A to Point B
When Lifting Objects off the Ground,

Make sure to Maintain your Lower Back in Extension. Ensure NOT to Round your Lower Back. In other words, your Lower Back needs to maintain the natural Lordotic Curve you see and feel, in upright standing position. When in this properly Extended Position, the Lower Back Muscles (Erector Spinae) will take the load, which is good.

If you do allow your Lower Back to Round, or, are unable to keep Lower Back in Proper Extension due to your Erector Spinae muscles being weak (not Strong Enough), then, you Load the Spine in a very Unnatural manner. This for sure, will lead to problems.

The Worst thing you could do, would be to pick up the Objects (Boxes) off the Ground, with a Rounded Back and then, proceed to Swing your Hips and Release the Objects, say into a Van/Truck. Again, an even more sure way to cause problems.

Also, follow proper Lifting Mechanics.

As much as possible, make sure Object is located at the Center of your Foot (less Moment Arm).
Lift initially with your Legs and then get your Back into it.
As mentioned above, Ensure you have your Lower Back in Proper Extension…Do NOT Round your Lower Back.
Take a Deep Breath (Belly Breath) and hold it, as you lift Object off the Ground…this will help protect your Spine.
It is quite OK to wear a Belt, when you Lift Heavy Objects. Will help with Holding a Deep Breadth, as mentioned above, thereby, providing more help to protect your Spine.
Make sure you are Strong Enough to lift each of these Objects. If you are not Strong Enough, such that your Lower Back starts to round, or if you experience any other intense physical discomfort, stop lifting the Object/s right away.