This is where I get my flange bolts. http://www.imperialinc.com/grp308.shtml The length depends on the thickness of the of your material. Drilling the holes out to 1/2 is an option if the holes are elongated. Channel would work, but it sure makes the job look unprofessional. How big a truck are you working on? If its a larger truck with a 34" frame, a cross member out of any old truck will work and look more like it belongs there.