I did the same removal and install on my 66 dart vert. It is a 6X9 speaker. Sometime later they went to a 4X10 speaker for the dash, maybe 67 ?
You will have to take out the radio, and heater controls in all likelyhood. I installed a dual voice coil speaker in its place. The retro radio outfits like custom sound etc sell these.
I also bought a pair of the kick panels that Dvert talks about above and installed speakers in them. I believe he was making and selling these a while ago. Very nice parts and they work great. Just need to pay attention to the depth of your speakers. First pair of speakers I bought were too deep and interferred with installation.