I do not own one, but I would agree with 67 and 68 Shelbys are under valued. It seems that there is more appreciation and value for a modified 67/68 Mustang fastback to look like a mis-named, lack of heritage movie car with funny hood, side pipes and stripes.
And then there are the early generation Corvettes that command way more value when modified with modern engine, wheels and sutpension than if original. Strange market. In another generation, maybe the values will be higher if all collector cars than have been modified with hybrid engines or all electric power and are self driven to the shows and on the tracks. Just add a crash dummy with a helmet.
.