So what you do is not mount the left side of the tab in the normal fashion but you drill a hole straight through the crossbar at the exact location of the hole in the mounting tab.
Second, you get 2" bolts (4) that will be used with the parts circled A. I don't know the thread particulars but you can take the tab with you (which is threaded) and use it to get the correct bolt. I would not recommend using stainless as they may not be strong enough. In fact, I am going to try to get some grade 5 or possible 8 bolts to replace the ones I have now.
And finally you slide the tabs right into the roof rails and bolt the entire thing down. Just be prepared to spend some time fiddling with the tabs in order to bolt everything down. They are a tad too narrow to make things easy (what else is new).
When bolting down you need to make sure that they end up sitting flat inside the rails and NOT on a vertical angle which would make the entire mounting problematic. The tabs need to be rotated horizontally so that they catch the underside of the roof rails absolutely flat across their surface.
Once the tabs are mounted perfectly, the whole set up is rock-solid. I was apprehensive at first with all the weight I was going to be putting up there but I did not run into any problems whatsoever on the trip. One thing that helps is that the tabs are aluminium and the underside of the roof rails are ribbed. These small ribs actually dig into the tabs slightly for added gripping.