I finally found the time to fool with this stuff.
I made large T-nuts for the mounts using stainless 1"x1/4"-3" bar and threaded them for M10 bolts. These fit the 4Runner side rails perfectly and are certainly strong enough and rigid enough, to a point where the mounts don't need the bent angle design. The Roofnest tent to sit on top weighs 160 lbs.
The T-bars themselves were threaded to M6 for $2 an end, can't beat that.
The truck is metric, no sense in mixing it up with SAE hardware.
BTW, all 3 were only out by 1/32"
While I was using my buddy's mill, I also made a bunch of ss T-nuts for use on the aluminum cased Roofnest tent.
Even the store bought plated ones started to show signs of rusting in just a few days.
Hope all is well with the OP, just couldn't wait any longer.