I am no expert like others here, but I am dealing with this problem in a different way. My 230SL has the original hole that is unmolested 1-3/4" by about 7". However, I can not find a resaonable Becker radio for this size. Most Becker radio you will find are the larger 2-1/8" size that were also used on sedans, the later pagodas, etc.
If I were you I would cut it to fit a radio I could find, that is, if the hole there is not the original size. If the original size and you are keeping it that way, then join the hunt.
-CD-
1967 230SL, 113.042 10
1983 300TDT, 123.193