dab68
when i restored my 68 fastback s most of the bolts twisted off when i took the grills out. i cleaned each of the outer grill assemblies on the bench drilling and replacing any of the pop rivets that were bad/rusty. for the pop rivets, i used 1/8 "short" alluminum rivets. they seem to match the OEM better. for the twisted off bolts in the outer part of the grill, i ground off and/or drilled out the broken stud. then i went to a local advance auto parts store and bought small oval shapped chrome clips. these are the ones that are usually green or black and have a needle sized pin sticking out on one side so the clip can snap into the chrome moulding. i then cut the this needle pin off of the clip and further trimmed it with metal shears so the clip would fit into the small area of the grill chrome surround. these worked very well when i put the grill back in the car and because of their shape, will not turn when you are tightening the nut up on the clip.
if you want to take some extra time, clean and repaint the black metal support bars on your grill surrounds while you have them on the bench. these grill assemblies are pretty involved manufacturing - nothing produced today even comes close to how well the old grills were put together 45 years ago. but if you take your time, you can restore these grills to look almost NOS. like famous bob noted, there are a couple of options for finding replacement parts that will work.