Make sure you read the service manual closely before attempting this. The bushings do not just get pulled into place until you think they're seated properly. The factory tool includes a precision measuring device, and the right-side bushing is to be pulled into place so that it's a certain distance from the front diff mount hole in the subframe. The left bushing is installed to where the thrust piece contacts the subframe (again, with the OE tool.)
I'm about to attempt this myself (with the $115 tool from Performance Products - can't afford $500 for the real tool) and I'm going to measure where the old ones are, and install the new ones to the same position. If you don't get them in the proper place, the bushings will be under strain, most likey causing premature failure.
Here's a link to the factory procedure:
http://www.w124performance.com/service/w124CD2/Program/Chassis/35-050.pdf
And, here's a post with very good, detailed instructions:
Got the w124 300d Diff mount repaired!
