I am working on something similar... with the idea taken from the "After The Battle" magazine... this magazine shows photos taken during WWII, and then compares it to a photo taken "today", from the same spot and perspective.
I am doing a road scene from the Battle of the Bulge, featuring the seige of Bastonge. I'm doing it on a swivel base "lazy susan" and as you spin it to one side, you see "Then"... rotate it , and you see the current view...