I used to drive a Euro 280SEL. It had a 3.46 ratio, and turned 3,000 at 60mph. Believe it or not, the torque convertor hasn't locked up all the way yet at those speeds. Get up to around 120, or so and it should be at around 5850 rpm (+,- 100 rpm).
You DO have a 6500 rpm redline, you know, so there's more flexibility than in the later M103 powered S-Class cars with the same rear-axle.
If the 280 topped out it's redline, it would probably indeed be in the mid to high 130's range, I guess. Maybe even low 140's.
I think at top end the ratio between rpms to speed is about 2.2:1. Meaning the speed you're engine is working (say 4,000 rpm) and the speed you are actually going (with 2.2:1 as the ratio it would be about 88 mph fully locked up).
There is a site with a 260SE going 202kph(about 126 mph) turning just less than 6,000 rpms. This car would be relevent to your car in gearing.
Click here for the picture