My point is that the stock early US and Euro systems without cat are full dual systems until the rear muffler. Both systems have a crossover pipe. If you wanted to go full dual, you'd only have to modify the last two or three feet. If you have a US 560SEL, evertyhing from the exhaust manifolds to the back end of the transmission will have to be redone for dual exhaust. It should have dual pipes from the second cat to the muffler.
86 300SDL (due soon)
81 300SD (for sale cheap)