my dually had one simular to that on it when i first got it and it kept cracking out and finally completly broke in two on both sides forcing the frame up against thde bed floor in the back and the top of the front of the bed into the back of the cab, but i think that that was mainly installer error. if you weld it well enough and maybe reinforce it a little more you should be fine.
You could cut down a regular notch to be that short. I don't think I would bother spending the money on those. Just get the 2x2 or whatever weld it to the top of the frame and box it in. Same thing as those and you know it's all welded right.
Dont know about squarebody trucks, but we set my notch up to be as low as possible in my truck and ran the bags behind and below the axel to make my notch cover as low as possible....