What is the condition of the battery terminals? Clean, corrosion free, and tight? Remember that dirty/loose battery cables can make a brand new battery worthless.
Measure the voltage at the battery. Post. Measure while cranking. Post.
Repeat the tests at the Main alternator B+ terminal.
2003 GT Convertible (sold & missed)
2000 GT coupe (Craigslist project. Fixed. Now my DD). Windsor to Romeo swap.