I am looking at doing this myself. from what I have seen and heard there are a few issues.
The swiss top has a solid frame that mounts via two tubes at the rear (slips in) and two bolts at the top of the windshield. The German style top mounts completely different and also has different points to secure the canvas at the rear. I have looked at a german 416 top and can only guess it mounts similar to that.
The side windows are also different between the swiss and german trucks so the swiss ones wont fit properly. From memory the Swiss ones are bigger than the german ones and mount differently (unlike the posts that the swiss use)
What I was going to do (seems the easiest way) is modify the swiss soft-top frame to give it the german profile, Luckily i have the canvas off a german 416 to use as a guide. Them fab up any mounts on that to allow me to mount the canvas. However I am going to put a full cage in the cab so when this is made I will get it made the profile of the German roof so the canvas will fit straight over the top of that (hides most of the cage and will allow me to fold the windscreen down with the roof on). Getting the cage made is looking like a cab off process though

to get it done without bolting it directly to the chassis.
As for the side windows..well i dont use them that often (living in Sunny Australia

) so its not a priority at the moment. If someone has a german roofed 404 and could snap a few pics of how it and the windows attach it would be great.
Thats about all the help I can offer.