Call Hertz CS and have them add to your profile "NO town cars or Grand Marquis". That doesn't stop them from giving you one but if it is in your profile, they are more likely to allow you to swap for another car.

When Hertz had the old Taurus with the anemic V-6, I put in the profile "NO FORDS". If I got the more powerful Taurus with a sub compact reservation, I usually kept the car. If I got the Vulcan (anemic) Taurus, I switched it out.
If I booked F, I asked for a different car. One place gave me a DTS instead of the anemic Taurus. ^ For the time, that car (DTS) had some power.

That was around 2008 or before.