Just do some measuring of the flat surfaces that aren't already dampened.
The good news for you is that a DeVille probably already has a lot of sound deadening done to it in the factory. Unless you're hearing some road or tire noise entering the cabin through the doors or floorpan, I wouldn't worry about doing the whole car.
If you have subs in the trunk, that might need some re-enforcement there.
I've done an entire car before, and it's not a lot of fun. That was a Honda, so it was probably only about 60% of the work you would have to do!
One thing I would suggest, look into the "generic" versions of DynaMat. You'll save a lot of money.
The foil-backed stuff is really good, but messy once it gets hot. In NY you may not encounter the same kind of summer heat I do down here. I had some tar-like glue dripping and oozing around in my trunk under the carpet. Once you put it on, don't expect to take it off.