I am in the middle of installing front springs on my '83 300D, and I would say it is the most terrifying thing of any sort I have ever done.
See, it's like this:
I tried to do the right thing and bought a center bolt compressor clone. Getting the old spring out was easy because the PO had cut the springs waaaay too short (which is one reason why I got the car for $400!). When it came time to put the replacements in, I could only grip 7 coils, and I didn't know about releasing the upper control arm, so it was a very, very tight fit.
I did manage to get the spring into the seat and the upper cup, but the coils above and below the compressed ones were at a great slant. So, when I put it in and loosened the compressor, the whole thing was curved dangerously--and, as you might sort of be able to see in one of the photos below, one side of the spring was not seated in the upper cup and ended up getting caught.
To top it all off, I was backing out the center bolt "a little bit" to see if it would magically pop into place, and--I don't remember exactly what happened, but--I there was less magic and more tragedy. I got to a point where the center bolt was out, and the spring is so dramatically curved, that I cannot get the thing hooked up again with more than 4-5 coils to compress. The tool doesn't compress enough to get the spring out, even with the UCA detached.
My only hope is to use a cheap rental to compress a handful of the coils below the ones I'm currently compressing . I'll be heading to the parts store in the morning to get that. I know it's not a very safe sounding option, but I can think of nothing else.
Yes, this is terrifying.