Here are a few photos of the Incline Cross 7 with Sussex Shaft Drive. Old Trek 1000 for comparison. NDS in some photos to show fame details.



Note the oversized, narrow bottom bracket with the chainstays hitting above the bottom bracket.
Right dropout designed for shaft drive to bolt to it. Left dropout designed so it descends below the normal position (so shaft drive goes parallel to the chainstay).

I have also seen older bikes where the shaftdrive actually replaces one of the chainstays.

Anyway, adding the shaft drive to an existing frame would be a major endeavor. Better to buy a fully assembled bike, or build 100% from scratch.