Good stuff, I highly recommend Red3 models, they are very crisp castings.
If you needed a more rounded metal cab roof, I'm guessing it would be possible to set to with a file on the canvas cover on the cab roof.
Intrigued by the differences, I found this site:
https://matarama.com/en/unimog-series-models-databasefrom which I extracted these pics

The top one is the 404, the bottom the 421, so yes, the 421 has more rounded bonnet but a straight top to the radiator grill, compared to the 404's rounded top to the radiator grill and straight bonnet.
The Red3 model seems to have the 404 grill and 421 bonnet!
