It was the water pump on my '76 450SL. I didn't have to remove the dizzy or the rad, but I did end up needing a new fan clutch. And, who wouldn't change the belts and hoses (and let's not forget the distilled water and antifreeze) Geez. It wasn't a fun job, but possible.

Although there's a weep hole on the top of the WP, there's another on the bottom. The culprit.

I pressure washed the motor before I started and made sure the car ran fine.

I flushed the system, then drained it all and cleaned it good.

I covered the fenders with pieces of shag carpet upside down. Not the best choice, but available and better than nothing.

I took the upper and lower hoses off, undid the 4 fan bolts, and the shroud and removed both. It was at this time that I noticed that the fan clutch, while basically working, the seal was hanging free around it, and the bearing had serious play.

I removed all the belts by loosening the adjustment clamps on each assembly (2 minutes total) except for the matched set which came off with the fanclutch

Using a 7mm small wrench, I painstakingly loosened the bypass [factory] hose clamps. Good light helps.

Using a 13mm socket and a 6" extension I removed the bolts around the perimiter and the one on top holding the dipstick. There's like 7 of them (most not easily visible and very close together). I had to rotate the flywheel to get the bottom 2.

I changed the thermostat over to the new waterpump while it was out, and using a 10mm wrench I took off the other water outlet.

Then I put it all back together using some GE Hightemp along the way.

I drove the car for 2 days to make sure all was tight. Then, I drained everything and replaced with distilled water (~2 gallons) and antifreeze (1 gal).

GMB Water pump - $100
BEHR Fanclutch - $192
Belts - ~$55 Dayco TopCog
Hoses - ~$65 All German repro's except the larger bypass which is Gates off the shelf (it's in the Gates catalog) like 42mm or so, cut to fit (less than 3").

I have the satisfaction of knowing I did it myself.