My '82's subframe broke about two years ago while I was taking my granddaughter to school, and I bought the reinforcing gussets from the local MB dealer and also got another subframe from the local junkyard so I'd have a good subframe to reinforce. As I recall, the dealer gave me a break on the price, only charged around $80 for the 4 small pieces of metal that probably cost under a dollar to make. He also gave me a copy of the TSB instructions. One point that was stressed was that the reinforcements MUST be MIG welded to retain the strength of the metal involved. It is necessary to prop up the engine and remove the subframe to fix it.
Performance Parts sells a modified subframe for around $600 that is much better, my '79 has one, and it looks to be much stronger than the reinforced one in my ''82. I took some pix of the parts involved, I'll see if I can locate them.

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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