Hi John,
I looked into this when I had to replace the broken subframe on my '82 380SL - Performance Products carries the subframe for around $650, and it might be available here on the forum also. I went to the local wrecking yard and got a subframe from a 114 chassis and installed the factory strengthing braces on it, which in my opinion aren't very substantial, and very spendy. (around $100) If you install the strengthing braces they MUST be MIG welded. My '79 has the improved subframe on it, and it is actually boxed in around the lower control arm tabs, a much stronger setup!
In my opinion, the newer boxed subframe is worth the money, and I should have gone with it. If you raise the front of the car up on blocks you can do the replacement job in around 4 hours. Be sure to have both new motor mounts and new subframe rubber on hand. It might also be a good idea to replace the sway bar rubber at the same time, since you have to disconnect the links.

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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