So to make a short issue somewhat long: I have a 73 Mach 1 that I restored about 19 years ago. I drive it all the time and it has been on 3 Hot Rod Power Tours. So this year on the power tour the stereo sounded terrible. I thought being as old as it was - 19 ish years - time to upgrade. So I purchased 6x9s to go on the rear package shelf. Previously it had like 6 inch pioneers. I also got a new head unit and door speakers.

So I remove the rear seat and pulled the package shelf. Well I only pulled a few pieces, since after 40 years it was mostly dust. There are 3 lightening holes in the package shelf, I made my template and cut the metal shelf with a dremel tool. All of my measurements were based on the lightening holes, since they looked symmetrical. Not so much, mine are 3/4 of inch off from the side wall from the factory. My speakers are going to have to be 3/4 of an inch off, since I already cut the holes, and I don't want to risk burning holes in the interior trying to weld in a patch panel. Just a word to the wise.

Arrrgghhh

So here is the finished product. I didn't like the factory carpet, so I bought some tuck and roll vinyl and covered the shelf. I bought a retro sound head unit, and Polk speakers around.

Another option would be to hang the speakers underneath. Offsetting the grilles on top to make them look symmetrical wouldn't affect the sound.
On a different year coupe I have I used some acoustically material meant for speaker cabinets that matches black cut pile carpet nicely. With it on top and the speakers underhung, you can't tell where my speakers are. Never did care for grilles much. Different way to skin these cats.
I've never been much on tuck&roll but that works there. Works really well, it looks good. Easy to keep clean too I bet.