Ever notice the how the thickness of a nut corresponds to the diameter of the bolt? (A 1/2" nut has 1/2" of threads.) That is because that many threads are needed to equal the strength of the bolt.
So, I you have 1/8 wall box tube, you need to add 3/8 of threads to the existing thread wall of the tube. Or weld a nut inside.
Fine threads are actually rated has having more strength than course threads.
My grade 8 chart shows 1/2-13 at 25,500 pounds minimum tensile, 1/2-20 at 28,800 pounds minimum tensile.
1/2-13 tap drill is 27/64 1/2-30 tap drill is 29/64.