I think you will need something more substantial for the W140. When I had mine done my indie used one that went inside the spring and compressed it completely the full length. It has to completely compressed in order to get the spring into the two cups even when the wheel is hanging with the weight of the alloy + tyre pulling the wishbone down.
Found this, a bit more expensive than i thought they were though. Could,nt find anywhere in the UK that sold them though this one does look similar IIRC to the Snap-on version. http://www.samstagsales.com/klann_spring.htm