Back in 2003/2004, Ford specified MERCON III ATF in the Tremec 3650 5 speed transmission in your Mach 1. Now, Ford specifies MERCON V for all applications requiring MERCON III.
Tremec, the manufacturer of the 3650 specifies Mobil 1 ATF OR DEXRON III. See page 8 of the Tremec 3650 Service Manual:
Note that Tremec specifies 3.8 liters of ATF.
Ford has since revised its fluid capacity for the 3650 to 3.0 liters:
A lot of guys run synthetic manual transmission oil, but I avoid it. According to a Tremec engineer I spoke with, some of the materials used in the 3650 specifically require an ATF.