The instructions I saw indicated that you choose the side with the most U-Bolt thread sticking below the nuts. Then clean the threads (wire brush etc) and put the mounting plate for the new track bar over the holes and fasten it down with nuts provided with the track bar kit. Then the Frame Bracket goes onto the chassis rail on the opposite side.
The DIY page is excellent, but I can't tell if Rich removed the original nuts and sandwiched his new brackets on top of existing or mounted them over top of those nuts like the kit I looked at does.
I haven't' seen one till now that mounted to both spring mounts, but Rich really succeeded at getting his actual track bar (aka Panhard Rod) parallel to the ground. The less angle the better, and I'm sure the commercial kits fall well within an effective range.
Rich's coach is P32 and new enough to have rear disc brakes so I don't know if Joe's is equivalent or requires a different approach.
Joe, you'd need to check and see how much of the 3/4" U-Bolt thread is available to add a plate and nut to. I'd think you'd need about an inch, even a little more.
If you DID have to take existing nuts off, you'd want your coach supported on jack stands along the frame AND the axle supported on a stand at the end where you were adding the plate. If you blocked up close, you could use a bottle jack to hold it up. Just block high enough that it can't drop the axle more than an inch or two if the jack suddenly releases.