I did that job a year or so ago. I think I took a grinder to the heads of the screws in the end as it was much much easier to get the whole thing off and sort it out on the bench. I had to make up some new captive nuts, but it was pretty easy to weld some stainless nuts on the inside of the flange before it all went back together.
I got a chrome grille from Martin when I did mine and it's still looking good.
Rupert
"Not tonight love - I'm varnishing me roofrack"
www.littleyellowcar.co.uk