Do you know what? I have bolted everything back together and it seems to be working better than I had expected at first. I removed a gasket between the booster and the firewall to make the booster-rod a bit longer in an artificial way so to say. The brake pedal is now in a position where it can push-in the brake light switch. (I had made this gasket myself but I had not thought about these consequences. The gasket is only about one millimeter thick, but this millimeter probably makes the pedal height vary by 5 times that. Besides that, I still believe that the booster-rod is a bit shorter than it was from the factory. The difference in rod length is probably done by mistake, when they welded the rod).
My only concern is that the pedal height may still be a little low (too close to the floor). Anyway, I will start with this setup and see how it works with the engine on. Thanks for your help.