In the past I have had 2 Superbirds, a '69 Road Runner convertible, and currently a '72 Plymouth Scamp that's a family heirloom.
While the values of musclecars are strong now...it's not going to last forever. If you look at the next generation coming, they are not interested (mostly) in the old American iron. It's all import-compact BS these days...and this is why musclecar values will drop. Just like cars of the from the teens to the 50's already have. Obviously, there are standouts like Duesenbergs and the like that will always command top dollar.
I have been showing national winning cars since 1989, and have seen the shows change drastically. It used to be that the "real" classics (20s, 30s, 40s) used to be very popular at the shows, with Model As for example, being present by the dozen. Now, the oldest car at a local show is a late '50s machine. Those that remember the cars of this vintage are either too old to care or have passed on. The same thing will happen to musclecars.
I still believe that you should never buy a car as an investment. Buy a car you REALLY like, just because you like it. If in the end, it makes you some money...that's a bonus. I restored my '72 because the car and I grew up togther and it means something personal to me.