The 2000 Mercury Grand Marquis 4.6 liter engine has a spark plug gap of .054". The recommended spark plug is an Autolite platinum AP764, or an Autolite double platinum APP764. The original Motorcraft plug, which lasted so long, was an AWSF32; followed by one of eight diffferent possible suffixes. If you want to use exactly the same plug as the original, take out one of the plugs, and read the number on it. The plug number and gap should also be listed on the emission information label that was originally located on the underside of the hood.
I hope this helps!