SORRY that was me that sidetracked a little with the 560sec..
on the topic it, the 722.358 is what came with the 300CE? What is the difference between the 722.351 that Motor Head has in the 300sel? And back to the original question as Jonathan mentioned it wouldn't make any sense to go with the 560 722.350 as it will not bolt to the M103. Sounds like the easiest thing is to rebuilt your 300sel tranny and beef it up, probably the most cost effective.
Off topic:
TT'ing a V8 either a M117 or M119 would be great and it has been done, but was leaning to the M103-04 as its much simpler. Less parts...2 less heads to worry about, 2 less cylinders and the biggest reason is the clearance. A TT M119 in a w126 would be a nightmare to get the manifolds and down pipe to fit and the driverside on my 560sec has the ABS/ASR unit that looks like would be in the way. With the M103-04 you can call the engine bay a living room..
Again sorry for getting this thread off topic.