I had a post, not the one you were thinking of, but showing rear speakers for a 78 and how the jack and tools installed correctly using a parcel tray cut. My speakers were low profile to suit the factory covers but for 4ohm stereo. They also allowed more clearance under them than the original speakers.
But you may have to add your jack and tools then measure clearance, I don't think I did. Likewise, regardless of what speakers you have, the large speaker insulated cover only fits on one side unless you clearance the side where the jack passes. It was designed for single rear speaker setup. There was also an earlier style jack holder bracket that was shorter but think it only moved the head away from the rear demister fan, didn't affect speakers or location.