Why the old Lincoln??

The newer cars tend to be a lot less roomy. Don't forget the backseat was basically his office. He needed enough room to operate and a really smooth ride in case he had to do some writing.

Why he choose a Lincoln instead of a Cadillac? There was an episode of " WKRP in Cincinnati " in which the radio salesman listened to some bad advice and changed his suits from polyester off-the-rack to tailor-made. His customers who included used car auto-dealers and the like could no longer relate to him.

The Lincoln is fancy but not so fancy as to alienate his clientele.

It is a 1988 Town Car, not a '70s model.

The Internet Movie Cars Database identifies the Town Car as a 1986 but Matthew McConaughey calls it a 1987 in an interview on the DVD. Of course, it's possible more than one car was used.

I was drawn to the novel by it's title. Then I found out the title derives from his car and it put me off for a moment. But then, during the course of the story, the car kinda grew on me.
I'm not a car enthusiast who could give lectures on the history of the model line, but the model used in the film, as well as how it was depicted, fits the story to a T.
Now each of the aforementioned enthusiasts can probably make a case why another model would fit in better. Though for the most part, I think these arguments say more about the person than the case at hand.

The Lincoln is so cool... old school but shiny and good. Authority car. Respectable. Still maybe expensive.

Anyway.It's old style.