I experienced the same thing when test-fitting the new gaskets on my 280SL. I left them on for 24 hours to see if they would give and shrink a bit, but no joy.
Then I received the latest Mercedes-Benz Classic Magazine. Inside was an article about replacement rubber parts for classics. ("Black Gold")
It stated that if rubber seals are too generously dimensioned, they can be sanded down to fit. I have not tried this on mine yet.
(The magazine also featured a nice 230SL in Ireland)