Cool your jets...
Most Fifties-era cars DO have the body offset from the frame to some degree.
My '54 Ford was offset an inch to the right side, the drivers side fender (and quarter, in the rear) was closer to the tire than the passenger side was. The reason for this is to allow for the motor/trans being farther to the right, for steering box clearance.
It's certainly nothing to get worked up over.
Offsetting the powertrain to one side was done (and still is) to solve a clearance problem and is an acceptable way to make things fit and work as intended. Offsetting the body was also done, as it would be obvious when the hood is open if the engine/trans sat to one side. Most people do not look close enough to see if the framerails are centered in the engine bay.
Offsetting the body DOES NOT affect the alignment or drivability of the vehicle. Alignment is the angles of the front wheels/rear axle in relation to the chassis, NOT the body.
I know all this for fact as I ran into having to center the driveline and the body on my '54 Kustomsledd project. I always center the powertrains I swap in, and when I did the air suspension I found out about the body being offset, one rear tire was rubbing inside the quarter when the car was slammed, yet my track width measurement said I had just enough room.
Unless you plan to spend a lot of time/effort/money to make it tuck tire, I would'nt worry about it. If the car does'nt drive straight then get it aligned, and don't sweat the body being offset...
BTW, De Ville's are made by Cadillac, not Chevrolet...