Bob, my 55 Special also has a knockout in the center of the package shelf for one rear speaker.
I did a nice 'mod on mine. Like yours, Bob, the former owner cut 2 speakers into the rear package shelf. So I was able to guiltlessly use his holes. I put 2 6x9s in the back, 2 4"ers and 2 tweeters in a replacement kick-panel, and an amp in the back. Here's the mod: I pulled the old radio, set up a piano hinge on the bottom of the speaker grill in the dash, and put a magnetic strip on the top, so it folds open and snaps shut. I built a panel out of MDF to fill the hole where the speaker used to be, and slid my deck into that wooden panel.
Except for the rear speakers, it is nearly invisible, and 100% reversible. AND I can blow the socks off the neighbors cat. Crank those oldies!