would it be possible to fit the final drive from the 20v n/a to the turbo gearbox to shorten the ratios that way and still have the strength of the turbo box
don't think so but no expert on 5 speed box. In the case of the 6 speed, not possibly to swap only the diff gearing as part of it is attached to one of the gear shafts, so have to swap those in as well
likely the na diff wouldn't be strong enough either
looking at my gearing figures above, the k coupe 20vt box would work for you, but rare as rocking horse droppings in the UK, you might find one being broken in Italy or Germany
Have no knowledge whether kappa saloon and SW 20vt have same gearing or not - the saloon is a lot more common than the coupe in Europe