I looked into taking off the cat converter on my 85 300D california when I bought it 5 years ago.
Went to two different dealers, they both said there was no replacement "pipe" to replace the converter.
Seems like this conversion was for the later models.
When I pulled the head to get the exhaust valve guides replaced, I cussed that darn thing. All the braces are hard to get to, not much room. Plus the starter has a heat shield to keep the heat from melting the starter from the extra heat that the converter puts out (no, I did not pull starter to do valve job).
And the starter is also different than the non converter ones, solinoid is down, because it will hit the converter. I know, I tried a non converter starter, solinoid up, hit converter.
If one was to make a replacement "pipe" you would still have to deal with the braces to take the strain off the manifold studs.
I just left my cat converter on, and have not had any problems yet.