Your 300 should have electric windows , right? . The W109 doors have different shaped panel inside the frame for Electric windows.
To fit a W108 Door you need to cut the window frame from the W109 door and weld it to the W108 door. the window frames are completely different . The Chrome is both screwed on and glued into place with mastic to prevent rattles and moisture getting between them. The screws are impossible to buy outside germany so be very careful when you undo them.
The correct rubbers are the only ones to use,but are $300 each these days. The off brand ones are hopeless .