From all of my research, I would fill it. the block is thin in the back as well as along the cam gallery. There are two forces at work on this block. one of them is a twisting force from putting in in gear and hauling. The other is from the top half of the engine trying to separate from the bottom half. More fuel and more boost increases this force dramatically. If you look closely at the block, you will notice that the area they crack is, is where the block curves in to create the separation for the water jacket and the crank case. The top half of the engine is pulling with the greatest force along this curve. The new blocks carry the force all the way to the bottom, no curve. I should mention that these blocks were created to be lighter and they are almost half the thickness of others. 6-7mm vs 11-12mm If your truck does not have the crank sensor in it. I would recommend that you buy any block you can find, except a 53. I believe that all should be compatible, but you need to do a little research I even looked into getting a 6.7, but mine has the crank sensor and I didn't want to change the wiring harness and the cam gear as well as the computers. If you weld plates on the out side, you risk transferring the load somewhere else and having it crack there as well as creating heat stress through out the block. If you try to epoxy some plates on, the epoxy will crystallize over time and your back to where you started. If you use the lock n stitch system, they will not guarantee anything, and you wasted your money. For the price of the lock n stitch, you could buy a new block. (not a good investment) The block filler will cost you about 140 and the brazing rod will cost you around 70. you can add some more plates to the side of the block, but I wouldn't do it without filling the block. The block filler will transfer the loads over the widest area possible, everything else I looked at only moves the loads to other areas. This is based on the research that I have done over the last few months. I talked with the manufacturers of everything I could find.