Unless he posted in the wrong section it's gotta be a 4.6 with a 5 spd. Either t45 or 3650. When I still had the t45 I went 150mph with the stock 3.27 gears, that had to have been past 3k in 5th. Then again, that was many moons ago.
Do you have the stock tune in it? Maybe it's governed exceptionally low with the taller gears?
If your GT came with 16" tires it cuts out at 130mph, if it came with 17" it cuts out at 150mph which should put you in the 4600-5600ish rpm range with 4:56. If it's cutting out before the Speedo reads at least 130mph you have some other problem than the Speedo out of calibration.