Well I took a look under the car yesterday, and putting a glasspack on the Grand Marquis won't work. Here's why:
The muffler is shaped in such a way that the pipe going in and the pipe going out are not in line, so one would have to use a blow torch to bend the pipe on an angle. That in itself is too risky. Plus, with a car that old, the place where the tailpipe meets the muffler is all corroded, so if I had tried to unclamp the muffler at that spot, the tail pipe could very well snap in two.
Unless you get some sort of custom muffler system for it, it doesn't look like there's a cheap do-it-yourself way to do it.
Also: The glasspack I tried was a Dynomax Thrush. Maybe another brand would fit?