I was going to say: pay no more than $100 (US). If I didn't already have mine, I would jump on it. Those U-bolts are about 1/2 of that price.
That will be a heavy box (so don't drop it on something you want to keep).
BudW
If he is, that might complicate things a tad.
There is an extra bracket to weld onto those plates if you going to use (or install) a rear sway bar.
BudW