Given the structure of Pure Stock Racing's Rules, you aren't allowed to do much with the motors to gain a horsepower advantage. You have to go with "free" horsepower, and that means a much lighter car. The rules usually "outlaw" special packages too like a police car would have.
It depends how you plan to race this car. If you were going to run a weekly "points paying" program, then I would have to say that the Crown Vic would be way too heavy. The races are just too short to overcome the weight disadvantage. If you were going to run the Enduro shows where the races are very long (200 laps or so) with no cautions, then the Crown Vic would be good because they are pretty bulletproof and will take quite a bit of abuse. You probably aren't allowed to keep all the fuel injection components (not sure of the rules at your track) but if you were, then it might be a decent way to go. Surely a full-frame car is the best.
There is a reason why you see so many GM products dominating the lower level weekly racing series. The cars are cheap, parts are easy to come by, especially body parts, and they are light cars. But, if you must have a blue-oval, then I'd look for an earlier Torino. Even they are pretty heavy cars by comparison. Those are the only Ford products that seem to run decent at the tracks that I frequent. I used to run a Torino and did pretty good but it's really tough to hang with the much lighter Cutlasses and Monte Carlos from the late 70's through late 80's. My car ran much better in the longer races. I used the Torino simply because I had it already. If I was to start from scratch to build a competitive car, I would definately go with a lighter weight GM car.
Probably not the advice you are looking for, but I felt the need to share because I've been there... which is also why I totally focus on my street cars now. It's the same amount of work but much greater glory.
Mark