I had the same problem happen on my 1989 300E. Later model vehicles (not sure which year) had a improved radiator that had metal in the flange to prevent it from collapse if it was over tightened.

The radiator shops I contacted could not replace the upper section, so I had to purchase a new radiator with the improved flange. So if you do end up purchasing a new radiator, make sure it is the updated one and not a new old stock radiator.

Rik