I would weld some vertical plate gussets to the mounting legs at the least. That first photo shows the issue very clearly, and explains why the RR’s are so spongy. There is no way you can cantilever all that weight on three little 1” tubes and expect it to be stiff enough.
Better yet...make some Franken-rails by keeping the inner portion of the GM sliders (with a far better mounting system), and simply cut & weld your RR outer tubes to the original GM inners.
That would also give you the opportunity to put some real powder coat on them, (although in your zip code, any coating is temporary).