The best thing to do (in my opinion) is to leave it as is and rebuild the car as you want it.

If you have bought the car for an investment for the future hoping that the value of a Sandman will bring you a windfall in the future, it will not happen (I don't believe you have).

Build to your budget and enjoy that car for its history and pleasure it brings you and others. As for authenticity, try and track down some original owner history, people love to hear stories of how the classic came to be where it is now.