For those of you contemplating replacing the water pump on your 500/560 SEL/SEC, I have just completed the task, and it was pretty straightforward. Shops charge 4.5 to 5 hours for the job. For me, it took a couple of hours longer (learning curve) so I thought I would briefly list the steps here for those who are interested:
1) Remove air cleaner assembly (for working space)
2) Remove radiator and fan shroud (both just clip into place) after removing water and transmission hoses. Cover tranny hoses with zip-lock bags to keep out dirt.
3) Remove fan (4 bolts).
4) Remove all 5 drive belts.
5) Mark approximate location of distributor rotor button, then remove distributor (6MM allen wrench)
6) Mark the relative position of both the crank pulley and harmonic balancer in relation to block. (Once removed, they will only fit the bolt holes one way).
7) Remove the 6 13MM bolts holding on the crank pulley. This will require the use of a mirror, as they are deep within the front of the pulley, and surround the main crank bolt). Tap on the sides of the pulley and remove from crank.
8) Using pry bars at the top and bottom of the harmonic balancer in a rocking motion, work the harmonic balancer off of the crank. (Requires 2 people unless you are adept at playing "Twister")
9) Gently move the dip-stick slightly to the left (allows access to one of the water pump bolts)
10) Loosen the hose clamp on the short hose from the thermostat housing to the intake manifold, remove the thermostat housing bolts and remove this housing and the thermostat.
11) Remove all 8 bolts to extract the water pump.
12) Clean off old gasket material from surface of block.
Installation is the reverse process. Ignition timing will need to be re-set, as distributor was removed and replaced. Pre-marking of rotor location get's it close enough to crank and drive.
I renewed the thermostat, the bolts for the pump and fan, and all drive belts. My hoses looked to be in excellent condition, so I reinstalled them.
As an aside, I have always had a fear of wrenching off bolts when doing this kind of work, so I used penetrating oil liberally and with lots of patience to prevent such a calamity.
Do not remove the main (center) crankshaft bolt, only the 13mm bolts. Work the balancer off the shaft by alternately pulling the upper/lower or the right/left sides. It will come, but not easily. Be sure to mark the relative location of the balancer vis-a-vis the bolt holes on the crankshaft. It will only go back on one way, as the bolt holes are not exactly symmetrical.