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These are pretty inexpensive and work very well. There are big advantages over using them in place of pop rivets, mainly the lower profile like original and the holding power.
look at the photo of the ash tray after re-assembly this is the clinch you should expect..
One thing with the grilles is pop rivets have a high profile and the rivet will likely interfere with the center grates as shown in the circled photo. (Courtesy BB-Dave)
I do not remember what length the ones used on the grilles were. I bought multiple lengths for all the pieces worked on From 3/16 to 3/8 so I had a variety handy. Grille, Ashtray, Fresh air doors, 2 speed wiper motor (uses 3 different lengths) and heater box.
If you were anywhere close I would be more than happy to let you borrow it and all the rivets you need as they came in packs of 100.