Pinion nut is usually straked down, you need to check that there isn't a bit of sheet metal "folded" down to hold it.
Tightening torque and rotational torque are two different things. What you want is at least 180Nm to hold the nut -- this is the minumum -- and more if that is what is required to provide the same rotational torque as the shaft had before. This is somewhat critical, as the "washer" you are replacing is a crush design, and overtightening will collapse it too far, leading to improper pinion to ring gear clearance.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!