Call me fussy, but if it were me, I'd get in touch with B&W if my kit was shorted parts. Assuming you bought it new and they are responsible, I'd suspect they could get the parts to you as quickly as you could track them down on your own. If not, they can probably supply them or, at the least, give you the specs.
If you do have to find them yourself, the loops are flat on top so they lay flat in the bed, not the half-round of the more typical U-bolt. Can't speak directly to hardness/strength, but keep in mind that hardness and strength don't go hand in hand. I might be all washed up on this, but I'd think in a breakaway situation I'd want something to the softer end that might deform but would hold, as opposed to harder that might just break off under a shock load like a trailer breaking loose. Somewhere around here there's some threads about that being why many hitches, depending on the setup, are mounted with Grade 5s instead of 8s.
My 2 cents.