'67 Barracuda Grille Removal HELP

Trying to remove the grille surrounds from a '67 Notchback header panel and need some advice and a sanity check. All studs/nuts are rusty as hell of course. The nuts appear to be 9mm, which baffles me. Moving on...

I have only successfully removed a couple so far. Every other one I've worked on, I get it to break loose (or snap off of course), but then the stud just spins. What the hell?

Wondering if I'm going to need to sacrifice the header panel just to be able the cut these surrounds loose and then work on each stud individually. Can't see how I'd get in there on the top sides of them with any cutting tools I have.

Any advice out there from anyone who has dealt with this head-scratching assembly before? Please tell me I'm doing something wrong. Thanks!!

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