I can't help you with your rear deck holes, but I have a Retrosound R-410N in the 4" x 10" hole in my 1968 Barracuda dash. I like it, sounds good. It's a dual cone speaker, so it gives a little stereo sound if used with an aftermarket radio. It can be wired monaural to give 8 ohm impedance to work with correctly with the factory radio. It clears the A/C controls.
There are people on this forum who make and sell an adapter to fit two aftermarket speakers in the dash. It's worth checking out. If it clears, I probably would have gone in that direction if I'd seen it before I got the Retrosound speaker.
