It sounds like the Passon 5 speed unit is geared (pun intended) towards guys with original 4-speed cars that don't want to cut up their cars but want an overdrive to make their cars more driveable on the highway.
Restomod guys are usually going to town modifying just about everything on 6 cyl or 318 auto cars so cutting up a floor to fit whatever trans they want is no biggie.Two different options for two different types of cars.