I did the mounts on my 1990 300CE many years ago.
As I remember it was a pretty straightforward job.
I used MB mounts and the new ones were about 10mm thicker than the ones that I removed - due to colloapsing / aging.
I guess with aftermarket parts sometimes you get what you pay for !

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1990 300CE-24 (Gone but not forgotten)
2002 SLK 32 AMG
2002 ML320 (Gone)
1995 E320 Cabriolet