Well, I'm pretty sure your car came with the Tremec 3650 tranny. So logic would suggest it should match the shifter that comes with the 3650.
Most speed shops can pick the correct thread size by knowing which year\type Mustang you have (Hence knowing which tranny comes stock...)

__________________
Uncle Sam
"What the hell is up with all the gauges?
Calling Captain Kirk, your ride awaits... Phasers on stun...."