The 66-67's are very desirable, they are just harder to restore. they also bring a lower price, but you can start lower as well. If parts were more plentiful restoration wise, they would bring more money. I have 68-69 model cars, but I get tired of them at shows. The Owners tend to be cocky, and they spend a lot of time competing with each other. A lot like the E Body Guys, sorry if that offends. Go to a Mustang show sometime, you will know what I'm saying.
The new group of attitudes? the 2009- 2015 Challenger Guys,,,,, I sold mine because I got fed up with them.
Sorry again!