Jeff, I was just comparing and contrasting the noise, vibration, and harshness (NVH) related sound deadening features between the two model years, not equating them per se.
Considering that the trunk underlayment appears to be the same material between 67 and 69, I put myself in the shoes of an NVH engineer and was thinking about how things change between model years and to a lesser degree running changes.
Next is speculation, based in part on observation. Yes, we need more evidence...
1. Trunk underlayment. It sure looks like the 67/68 and 69/70 material is the same. It also appears that this was included with the interior decor option of 67/68 and the Grande/Mach options of 69/70. I have not seen this on the 65/66 models, but have worked on far fewer to say.
1a. There is a question raised by Bob G. that the "extra" material on the trunk flats as shown in the 67 picture in the original post may not be original, and that was more of a 69/70 thing. A few extra pictures here. Sure looks original. But like Bob, this is the first of this type that I've seen on a 67/68.
2. Floor underlayment. Could it be that since NVH was a growing concern, that the floor underlayment on the "deluxe" models were built with either different material and/or had additional coverage? I have no specific observations here, just asking the question in the spirit of the topic.
3. Undercarriage sprayed sound deadener. Based on what I have read and observed, in 69 for Grande/Mach options there was additional material applied below the vehicle. I have never seen this on 67/68 so was stating that this appears to be an engineering improvement made for the new body style, and does not apply to 67/68.