Yes it is true, a 300E style fan clutch will work on the 603 motor.
I just installed mine finally today here and it works as expected. Now the
only catch may be that I have the updated plastic fan but I do not see the older metal fan not working with the 103 clutch. Someone will need to try this, if I can remember I'll take the metal fan from my parts car sometime and try it with my dead M103 clutch out in the garage.
You may ask why I chose to do this? Well after research both lock up at the same temp (~100°C) and the biggest factor is cost. M103 clutches are around $100 less than just the new 603 clutch. That's a big difference.
Just an FYI to all 603 owners.