I did the job on my 72 250 with the tool below - with the heads on. Wasn't really that difficult once I got going and figured it out. I used compressed air to keep the valves up and in place. I am including the link and the description of the tool - probably doesn't work as well as the Mercedes tool, but it's a lot cheaper.
http://www.mytoolstore.com/kd/kdengi13.html#912
Ryan
VALVE SPRING COMPRESSOR
KD 3087 $49.95
Tool compresses valve springs on most overhead valve engines, including GM, Ford, Chrysler, Nissan and Toyota. Tool can be used with cylinder head on or off the engine. Jaws adjust to fit over spring retaining washer, and an adjustable handle allows for easy clearance in tight areas. A GM adapter is included. Use 3269 Fulcrum Rail if overhead cam is not available. Use with 901 or 2992 Air Hold Fittings to keep valve in place. Caution: Vehicle must be placed in park/neutral before applying air pressure, otherwise vehicle may move. Keep hands clear of fan, belts and pulleys since slight crankshaft rotation will occur.