Unless 6th gear (T56) uses the Viper-Spec .50, I don't see a 6-speed improving MPG enough to make a difference (specific to MPG, but from a strength standpoint I'd choose a T56 all day long). The 3650 in the Mach 1 had a .62 5th gear and so did the T56 in the Cobra. I am not sure what spec the GT500 and newer GT's have for their respective overdrives. I thought the GT's 3650 had a .64 or a .68.
EDIT: Also dependent upon rear axle ratio. Mach's have 3.55, new GT's have 3.31 or 3.55, etc.