With a stock size tire and stock 3.92:1 diff I think, you should be turning around 3200rpm @ 60mph. Autos a bit more as the converter slips, they are non lockup units in these cars.
Remember, the car has 62hp I think... maybe it was 67hp other years like my old one. It is made to dork around town, not for top speed runs on the open road. Would I drive it at 70mph? Sure, will not hurt a thing, 80? Eh, maybe some. Not a chance in hell I would swap to a taller diff than what is factory installed. Not enough power to move with the A/C running.
My 300SEL has a 3.46:1 and I think this would work fine with a 2.88:1 and first gear starting. But eh, I am accustomed to it turning 3400rpm @ 70mph and around 5200rpm @ 110mph. Motor is fine with this lifestyle.
Lash those valves, have the injectors redone, and enjoy the roads.
