Any shorty headers for a Mustang will fit on a Crown Vic. However, the flanges for CV exhaust are a bit smaller than those for Mustang exhaust, so it's not a perfect fit. You can get them to go on there, but you need to stretch out the bolt holes in the flanges a little bit. Even then, they'll leak slightly, so if you can afford to do so, your best bet would be to get a new exhaust system made. If not, you can certainly put 'em on stock exhaust as I described, like I say, they'll just leak a bit.
I have stock Mustang headers on my car. Here's how they look in the engine bay:
And here's what the fit looks like on the stock flanges. Not terrible, but certainly not perfect, either. Of course, this is without the flange piece bolted up, but you can see how the fit is:
1989 Ford Crown Victoria Police Interceptor
2bbl Carbureted 351W V8, ported and polished E7TE heads, Edelbrock cam
AOD Transmission with B&M Shift Kit
3.55:1 Positraction Rear Axle
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