Which rear speakers for a 67 Valiant?

It's my girlfriends car and I want to get her a stereo system for Christmas since it only has an AM radio right now. On my 71 Duster, there were 6x9 holes in the back tray, so I just had to cut the holes in the cardboard type covering to install them.

Her car has built in factory looking speaker grilles in the back tray covering. She has the car so I can't really go look at it for another week or so but I'm assuming there are also 6x9 holes cut into the back tray there for speakers. Is there a type of speaker out there that I could mount in there without cutting the built in speaker grilles? Some kind of speakers that mount sorta low profile and wont stick out like my Duster 6x9's do?