My first ever Mercedes was a 1968 250/8 It had thrown a rod through the block so I sourced what was supposed to be a replacement engine. Things like the oil cooler lines didn't fit but I was able to get the head from the bad engine on the replacement engine and get it running. It never did run right so I threw a set of brand new Webers on it and still had trouble.
Only thing I found that might have been an issue was that the timing chain could not be indexed properly It was about a half tooth off no matter how much I fiddled with it.
Here's a link to an online manual for the Zeniths,
Mercedes Benz Zenith Carb Manual | JaimeKop.com
Michael