I would think extra length of 30mm would work better when you pre-install lower set of bolts in the frame then it would be just a matter of hanging the rail using the open ended mounting holes before installing upper mounting bolts
picked up a set of rock rails without hardware few days ago and tried M10-1.5 X 30 but would not thread
I'm thinking threading maybe JIS which would be M10-1.25
there is also M10-1 thread but I doubt they would use that for frame