I enjoy owning and driving something that most other people don't. At the same time, I don't spend a lot of time thinking about what it's worth, that is until I start thinking about making a major modification like cutting the floor out of it for a 5 speed. :-)
Reality is, even if they bring full value (one listing I've seen for a #1 condition 74 Duster 360 is $16K) there is little chance you will be able to bring it up to that condition and spend less than they are worth to do it. Don't buy and build a car as an investment, it will rarely ever pan out. Buy and build it because you like it. If you don't like it, don't build it.