You’re hunching over too much, especially toward the end of the set. Look at your neck and head–they’re parallel to the floor. You need to stay much more upright. (A little forward lean is fine.) Think about getting your elbows to move back and out.
Watch this guy. Note: The first exercise he does is a shrug, which is not what we’re going for. It’s the second variation, a little more than halfway through the video, that you should emulate:
I don’t use a belt much–but then, I don’t go very heavy on anything anymore. It’s really a personal preference thing. Why not try it both ways, and see if you feel like it helps?
You’re way too upright on the Pendlays. Remember, this is just a deadstop barbell row. Your torso should be flat, and almost parallel to the ground. Look at the guy in the video embedded in this article: