bought both grilles, the silver chev bar just unclips and theres a plastic bar underneath it.
with the ss grille, theres a groove along the top edge top the bar and around the edge of the badge i used a hacksaw/bandsaw/dremel to cut it.
The actual ss bar ends up longer than the chev bar, so all thats needed is to trim each end of the ss bar on the same angle as the chev surround. but leave it a little bit longer. measure it about 30 times before you cut it and dont trim too much off at a time. it probably wont fit perfectly, but its hard to see in the photos.
I posted a photo of the remnants of the ss grille..
the way its attached together, i superglued 6 4mm nuts to the inside of the ss bar then i drilled 6 holes in the chev grille surround in the corresponding spots. then i wound 6 nuts onto 6 screws, put some loctite on the ends of the screws and screwed them into the nuts on the ss bar though the holes on the chev surround, just enough so they grabbed. waited for the loctite to set then i wound the other nut on each screw down so it pulled the ss bar down tight, then i loctited those nuts. solid.
Then i superglued and epoxied the bottom of the holden badge to the new surround, just clamped it together while it dried,
the top of the holden badge just touches the top of the new surround, so i glued it there too.
there is a difference in colour between the grilles, so i painted it with vht vinyl dye.