If you like it, it matters not what we think, but here goes. My thoughts are evident in the questions, but for the record I once owned a Ford F-250 with the 460ci V-8 — 10 mpg and I loved it.

How many miles on the odometer and is it the original engine? Which engine?

How far will it take you on a single tank of fuel? Really 3/4 tank, the last 1/4 is for emergencies.

Keep cash available to keep the tank full if you need to bug-out from a hurricane.

How will the availability of this largish and very capable vehicle change your bug-out kit?

Edit: a few answers are available at:
1989 Mercury Grand Marquis 1gen Colony Park
and
1989 Mercury Colony Park GS specs

302ci V-8, 18 gal fuel tank, 14-20 mpg depending on how you drive (20 hwy, 14 combined?)

As a comparison, my pick-up has a 20 gal fuel tank and the worst fuel economy I’ve seen is 17mpg. On the hwy I get ~22 mpg.