Are the 4.6L and 5.0L Engines in the Town Car Worth the Upgrade?

I'm considering upgrading my Lincoln Town Car's engine to either the 4.6L or 5.0L powerplant, but I'm unsure if it's worth the cost and effort. Has anyone else made this upgrade? What kind of performance gains can be expected? Are there any specific modifications that are recommended for a smooth integration with the existing transmission and drivetrain?