Jounce bars question

[Stg1Regal]

I've put a GP cross brace and " Jounce" bars, for triangulation bracing, on the front of my 82 Buick Regal.

I've notice the other day that one of the jounce bars was loose at the mounting point on the engine crossmember, went to tighten it up, it won't.

It gets snug then loosens if you go to tighten down past that point.

Any ideas on how to get soild tight?
There seems to be no access to the inside of that to put a nut.

Thought of using one of those fender U-Nut clips...... Ideas?

Thanks