For a fleet car it might be cheaper in the short-run to pick up something else, but there's nothing like the good ol' Body-on-frame Town Cars. There's a reason why every major fleet in the US uses the old Panther cars; because you can swap out a rear-end in a lunch break.
If I was a business owner I'd be real hard-pressed to change from what's VERY tried and proven. Particularly with something that I know is going to significantly increase my variable operating costs over what I'm used to.
And though I doubt it's of much importance, I'd check on whether the V6 300 is indeed any faster. I know that the 300 Touring is the slowest accelerator in its' class, even slower than the 200hp Duratec in the Five Hundred (when attached to CVT) in reviews.
Oh, and for a counterpoint, the Town Car is slated to end production next year. So, in the long run, parts (those that are unique to the Town Car) might get more difficult to come by. But I imagine 99% of the Town Car parts are interchangeble with the Crown Vic.
