Here's what you get when you look at something like the UPR pieces that you just pictured....
A warm and fuzzy.
Just because they fit does not automatically make it worth your while to install them. The wall thickness of those connectors look to be about .o60"
Buy them, put one end in a vise, grab the other end with a pair of channel locks, and see how much force it takes for you to twist them.
I bet they twist like mad.
"Something is better than nothing" is all you'll be doing for yourself when you add a cheap set like that to the floor of your car.
SF connectors in these cars are a big deal. They can go from adding the "modest" rigidity that you'll get from the addition of something like those UPR pieces, to making the chassis so rigid that you can jack up the whole back of the car from one side. The more rigid you make it, the less of an instance of chassis twist. If you control chassis twist, the car handles better, it 60 foots better, it rides better, it doesn't rattle every damn thing loose, every time you go down a rough stretch of road, and the floors don't rip.
If you can weld, make your own..it's probably the easiest project to make a set of SF connectors on the planet.