Really depends on how nice and tight to the trans you want the floor to be, what kind of trans, how much you want the floor to intrude into the passenger compartment, etc.........
In this car it was done with the trans in place because of the low ride height and I didn't want the tunnel huge. You can see that the metal is literally molded around the transmission. This is really an extreme of what to do, as most trans/engine/cars dont require this much work (starting by cutting the tunnel up and mocking up with cardboard)
