I'd put my 04 Mach1 up against that car, when it was bone stock and whoop it's add big time. Standing start or a roll! A 70mph roll would be a drop to third and to 150mph in nothing flat. That being said, a new GT but make my old Mach1 go pee in the corner! LOL!
If that were my car, since it's not matching, I'd put a modern automatic or better, a new 6spd stick. Tremmec came out with a totally redesigned 6spd. Beefier, better syncs for 7500 butter shifts, more compact to fit more swaps, multiple shifter mounting positions........ect. Bad ass tranny!
I'd love to have one behind a modded high winding Coyote in my Mach1. That would be a FUN car!