Hi JNP,Engine in your vehicle is incorrect, should be M189,all Aluminium straight 6cyl. Don't know where u r,but if in U S A, 3.5 ltr engine never officially imported, only de. compt 4.5 ltr. If Eng. No. is correct comes out of a 280 SE 3.5 rated @ 200HP DIN when new. In relation to upshift probs u must identify which autotrans is behind your M116.980, K4A025 300SE, K4A040 or W3A040.
Re this car: that 3.5 was not the original engine and as pointed out correctly, it should have been the M189 3.0 liter motor.
Just a moot point in regards to the topic in the thread, but worth correcting:
The 3.5 was officially imported to US
for a couple of years (1969/70 +) before the 4.5 came out(1971/2). It was in the 300SEL 3.5 , the 280 SE 3.5 (Coupe only) and maybe some early 350SLs (official U.S. version). It is correct however, that the 3.5 sedans were NOT official US cars.
Re: transmission, it probably is the one that came with the motor (3.5).
It's worth checking, if the body, mainly the transmission tunnel was modified to fit the bigger transmission, especially the automatic as in JNP's case.