I put a 380SEL rear end in my 1984 300SD, at 2,500 RPM's I am going down the freeway at 72 miles per hour, I also have 205-70-14 tires on it. My third gear is now the same as my fourth gear use to be, Fourth gear is my overdrive. I love it!
The only difference was the yoke, the 380SEL yoke was larger, So I replaces it with the 300SD yoke.
So, if the 126 fits the 123, this would be an option and the 450SL, I would think if you do some measuring, you may come up with your answer.