Your 124 has a diff which is carried in a sub frame . the whole five link assembly can be lifted up into place and the mount position determined .you may need to shift the spring location but that that not a problem. I would go with this approach, (a freind has done it in a W111 coupe) because of the better ride you will get.
the M103 will be an easy fit intro a W114. it's not as big as the M110 and although it lays over ,the amount of space reqwuired is minimal. I have seen V8's in W115's with cowl brace so a skinny six will fit fine. The big automatic wont fit in the tunnel though. I would go a for a heavy duty 5 speed although you could possibly have to import one from Germany where they are common or France,where they are really common.