6.3 cables are very expensive. I made my own cables and bought new spark plug terminals with a set of 90 degree ends for the distributor cap. Whatever you do, keep any carbon core plug wires out of your system - too much resistance in that stuff.
Just make sure that the spark plug wire ends aren't too high in resistance as Benz Dr. explains. Also, buy non resistor type sparking plugs. The non resistor type plugs are still available from Mercedes.
The original spark plug wire ends were a tan/orange phenolic plastic material. So it you're going for originality then find a set of the original ends to screw on to your new high tension leads.
Primo, the high tension leads are not interchangeable between the 6.3 and 6.9 from a technical standpoint. It's mostly the distributor cap ends that'll rain on your parade, but they do screw off.