You can see how much tighter the fit is compared to the first pictures. GKTech in their how-to videos shows that you can also do the installation welding with a Mig welder without fitting. This sort of welding is actually better than the gloppy OEM welds on the cross member so it is not detrimental but Zac is a perfectionist craftsman.
Now that is a nice, tight-fitting part!
To hold the reinforcement in place for welding, it is bolted in place via the differential mounting holes.
Here is a final look at how much tighter the gaps are after fitting, compare this to the first photos before fitting and see.