opinions are fine- i have seen and fixed quite a few of these and other frames, i have used combination bolt and weld ones, not just bolt ones though. Bolting is a repair alone may be fine but i would go for strenght and do both, either bolting or welding done wrong can weaken the fram rail. A good weld job makes all the difference in the world though, with not enough penetration there is definately going to be problems, With a proper weld it could be a repair that never has to have attention again. Sometimes the cracks can extend beyond the brace or drilling to many holes in a fram can weaken it, it depends on the design, i have seen some custom made braces that had bolts going through the frame but the crack is already beyond the bolts all that does then is space the box off the frame and bolt a plate of metal to the frame, bolting the plate is a good idea, if the crack is on the corner of the frame and the brace you get doesn't wrap around that corner and hold the two "pieces" of the frame together the crack may get bigger. If someone shortens a frame they usually dont just rely on a plate with bolts in each piece of the frame, it is often welded and bolted. I could see using a bolt type only, but the crack in the frame should definitaly be repaired before with a weld and cease holes drilled on either end of the crack, otherwise it will probly spread. (seen it the whole box came loose because the frame rail cracked under the brace and allowed the bolts to move a bit then they loosened and cracked)
I learned of frame repairs from semi-truck repairs and before i was a technician, and at school in UTI. I was taught to use both. In this application i figured you have four bolts holding the plate firmly in place and two bolts holding the bottom plate in place, but the two plates in the kit should be welded together and to the frame. (does your kit have 2 plates) If the plates are not welded together there will be a vulnerable section between the two on the corner of the frame which is where mine was cracked with a 5 1/2 inch crack. Every situation is different and calls for different repairs, the only way to be happy with it is probly to trust your own judgement after considering others advice.