A 2v 4.6 could be a good swap, but the pre 96 4.6's are JUNK! I have a Crown Vic fetish and can tell you all the difference between them. If you are looking at Vic engines get anything after 2001. That is when they went to the PI stuff, also the 4.6 in the 2004 and later cars are rated @ 250hp, exacly the same engine as the 260hp 'Stang engine.
On the 94 engine, if it is still in the car and you can hear it run, and it isnt smoking to beat the band (pre 96 have problems with valve seals) and you can get it ALOT cheaper then buy it and tinker. But the throttle body is at the rear of the manifold, and the intakes flow like crap. also they are only rated at 195 Civvy and 210 Police HP. Sorry for the disjointed post... I'm tired.