Power to manual steering

if they did floor shift automatic steering column shafts for cars with manual steering one of those centre shafts would work in a car that was previously a floor shift automatic car with power steering, provided you get the coupler as well......
the power box uses a different spline to the manual box
and as you know the manual box is shorter so you need a longer shaft

other option is the expensive but easy to fit spline adapter from mancini or firm feel
and although you have dismissed it there are benefits

no dismantle of column and no new coupler needed
it just takes up the gap now left due to the manual box being shorter than a power box

Dave