I met John Snead, Cartographic Manager at the Louisiana Geological Survey at the cochon de lait we attended today. I mentioned my problems with the 15 minute historic quads, and he told me that the answer lies in the properties of the projection. Specifically, the meridian, lat/lon and other properties. I need my projection to match.
He suggested that there may be metadata available on the maps, which would be contain the information I'm looking for.
I'm going to e-mail Joe Holmes at DEQ to see if metadata exists on these maps.