6-speed tranny in a '69 Mustang

As we all know, the six-speed transmission is commonly found in many of today's manually-shifted cars. I've read countless write-ups in magazines covering cars, especially classic Mustangs, with five-speed transmissions. Why haven't I seen any classic Mustangs with six-speed boxes? Are there any special issues WRT putting one in a classic Mustang (especially my '69), aside from the expected bellhousing, transmission input shaft, transmission crossmember, and possibly transmission tunnel clearance issues? Any help/insight would be appreciated. Thanks!