I'm unclear here, are you spacing the stock end pieces off of the frame rails with the tube stock, or?
If you are spacing then you really want the tube sides to be parallel to the bolts (i.e. the bolts run down through the center of the tube). Unless the wall thickness is huge drilling through the sides of the tube will squash it when you tighten the bolts.
Same, btw, is true of the frame. I welded sleeves into the frame. I drilled an 8mm hole all of the way through, and then drill the outside holes, only, to 3/4" I made the sleeves from 3/4" bar stock. Bolted them in place, and then welded them to the outside frame wall only.