You do want to be careful when tightning bolts that go all the way through the frame so as not to 'crush' the frame. If you have access to the proper tools the way to do it is to drill LARGE holes then insert a piece of pipe (sleeve) thats just a hair bit bigger that the bolts and weld it on both sides. (Known as a bung. Not BONG )
This provides a very solid mounting place. Make sure to coat the bolt with heavy grease or anti-sieze to prevent it from rusting solid.
Just so you know, my ARB FOR the X didn't fit very well and we had to enlarge 2 holes then use a pry bar to get it in place. Had to do the same thing to another KYFXC members ARB.