Very minor thought this morning, but a washer under the bolt head will distribute the load more evenly. It's not uncommon to find the underside of a bolt is not smooth so would cause points of high stress.
Of course, as there isn't much space, a washer might not fit, but even a cut down washer ought to be better than none.
The point about anti-tramp bars also makes sense. While you need to be a bit of a loony on the road to fully appreciate them, Woodstock was a far nicer car on the highway after all the competition modifications were carried out.
