Yep #1 should always be another rig, always.

The kit is a good start for something that is always going to be in the vehicle, but I would add another kit that you must pack for the trip. This would include overnight clothing and food for atleast two days. It is the day trip/cfouple of hours outing that people end up getting stranded on. When I hit the trail I always pack enough food and water for 2 days more than I plan to be out, something that has carried over from my backpacking days. Of course I don't mean pack full course meals for a day trip, like when camping, but you should pack enough caloric items to get you and your passengers through 2 extra days.

You should also pack those parts that are prone to berakage, i.e., steering linkage items.

I would also add some welding rods and a mask to the kit, with instructions on how to wire up the 2 batteries for trail side welding repairs. For this you will either need to carry an extra battery anbd 2 sets of jumper cables, or make sure your wheeling buddies have a set of jumper cables w/them.

You can also add duct tape to teh tire patching portion. We fixed a 2" slice in a sidewall(second on the same rig) using duct tape. You take a section or the tape, roll it so the sticky side is out and stuff it into the sidewall slash until you can't stick anymore in it, then fill with air, if leaks stuff more tape in it. The 35" tire we fixed this way held 15psi and worked to get the truck 2 miles back to camp, and was still holding air the next morning.