I suppose a person might be able to do the job with the subframe in the car, but it would be a lot easier to pull it out, clean it up properly, and replace the bushings and motor mounts at the same time as reworking the subframe. That's what I did on mine, and the whole job took under 5 hours to do, one person only. The subframe I got was from an earlier sedan with a diesel engine, so I had to drill access holes for the motor mounts, plus I switched out the control arms on both sides also. I wire-brushed the area before I welded the reinforcements on to ensure nice welds. I also sprayed the subframe before reinstalling it, first with red lead primer in the areas with bare metal, then gloss black. Of course you can do as you see fit, but for me it was much easier to pull the subframe out of the car to work on, and also I switched it with another one due to the damage the one in the car had. (the left rear control arm end tab broke completely off, and the control arm was dragging on the pavement.)

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Richard Wooldridge
'01 ML320
'82 300D 4.3L V6/T700R4 conversion
'82 380SL, '86 560SL engine/trans. installed
'79 450SL, digital servo update
'75 280C