The fuel distributors on those are very problematic and frequently cause idling issues. Have you tried switching the fuel lines around to see if the problem stays on the same cyl or stays with the same port of the fuel dist?
Usually the M119 can tolerate a considerable amount of vaccum leak from the intake without a well definded miss.

If after everything has been checked and re-checked and still without resolution, it may need a valve job. As a Damn good M/B tech I can tell you that the Very Single BEST tech I've ever worked with reminisced about M119 needing valves and seats ground to cure rough idle problems when they were still under warranty. The guy was a huge pain in the butt,l but he was never wrong...ever, literally one of the best in the nation.