| Well done on the post and images.
Great description of what will remain as a potential failure point on these cars as long as they are still rolling.
A reinforcement job is probably prudent action to avoid future failures. I could join the chorus and lambaste Mercedes for their poor design or inconsistent execution of a major safety component, though I have been reassured by a SoCal MBZ dealer that the incidence of cracking were never very high and the total failures could be counted on the fingers of one hand over the course of some 25 years.
While I don't see any subframe cracking on the two R107 examples I have on hand, the updated subframe, ala 560SL from '86 onwards, looks to be easily approximated. A new subframe from the usual sources is an $800 proposition. Given the time commitment and cost, it's not worth it if you have an otherwise undamaged subframe, or a damaged subframe on a later car not covered by the recall (or just have a local dealer that doesn't care to fix it).
From the images, the gusset kit doesn't appear to have boxed in the tabs at all (two pieces a side, not a third member to join the gussets). The eyelets for the control arms appear to be another point of weakness unaddressed by the gussets. The particularly bad (rusted and cracked?) image could probably only be salvaged by cutting off the top of the tab and fabricating a new eyelet to weld on. For the boxing in that the gussets don't provide and get closer to the upgraded part without the same cost/complexity, another piece of flat steel, bent over sharply at the edges (or just plain flat and welded at the edges), would do a good job to bridge the gussets together. The two drainage holes seen in the image on the upgraded subframe shows the boxed in tabs should be retained.
From the images, I'd say the gusset kit is nothing special. With dimensions handy, you could probably fabricate these fairly easily and actually improve on them. Anyone have precise dimensions?
I have an acquaintance who is a professional welder that is still trying to convince me that he can TIG weld almost anything in-situ. I posed it to him today after seeing this post and he confidently stated he could weld up reinforcements with subframe in place. That would of course be preferable, considering the labor cost of disconnecting and dropping engine/trani out on the subframe, then dollying the engine/trani to work on the subframe all by itself.
While I would like to say I trust his work overall, welding isn't my expertise. If anyone has any further thoughts on TIGing the reinforcements in-situ, I'd like to hear it before volunteering my otherwise serviceable un-cracked subframe. If I take the plunge, I'll post images of the retrofit to '86 era "upgraded" boxed in subframe axle-carrier configuration. | 
