Need some help
Poblem: The Engine does not take gas properly and stutters. At constant throttle position you notice that the engine runs rough. At full throttle it gets a little better.
The spark plugs are dry, but black. Does indicate that the engine is too rich, or not? When cleaned the spark plugs, he is somewhat unwilling when starting and even patches sometimes quite properly.

What i did:
Engine was completely overhauled 4 years ago. Since then hardly driven because of resto.
- Drilled / honed for new oversize pistons
- Crankshaft ground to oversize bearing
- new camshaft/bearings
- new valves incl. guides/tappets. Head planned
- new connecting rod bearings 
- new timing gears
Carburetor overhauled by me, with repair kit (gaskets, jets, float needle..). Float level checked according to manual. Throttle Connector checked with Metering Rod Gauge
Automatic Advanced Mechanism checked
Compression checked - all 6 cylinders between 100-108 PSI
Valves adjusted
Ignition timing checked
Distributor Point Gap adjusted
Idle adjustment according to manual - once with vacuum method and once with 1 3/4 turn basic position and adjust to idle 

But somehow all this did not lead to success

Did the engine run OK 4 years ago after the rebuild?

Gene's question is very relevant. If it ran well after the rebuild then we need to look for what has changed.

My first suggestion is that the carburetor needs cleaned/rebuilt again. If the engine has been sitting a lot either the gasoline has evaporated and left varnish in the carburetor passages. Or the gasoline evaporated and let all the gaskets and seals in the carburetor have dry out.

It does not sound like an ignition or engine mechanical problem.

No, as far as I remember it was already running badly before that

If it were mine I would check the valve adjustment and lubricate the valve guides and then do the suggested carb. work above.

First thing that comes to my mind is that the fuel is old. (yes I read that it was also running bad just after you rebuild the engine, but the fuel might have been old back then too).

I have very bad experience with german/european fuel older than one year.
When I started my car after two years in the garage, I could hardly keep the engine running, it was running really rough as if the mixture was way too lean. Simultaniously it was smoking black as if it was running way to rich. With fresh fuel, everything was OK again.

I saw the same thing on another car this weekend, old fuel (which we got for free) was put in the tank and the engine didnt run anymore and black smoke was coming from the exhaust. After the old fuel was pumped out and the car was refilled with new fuel it ran perfectly again. We didnt even need to clean the carburator, it was just the fuel itself.

You could try to run from a jerrycan with new fuel (if possible without any ethanol) and see how it runs. This is an easy way to check if the problem is the fuel.

Now the car is back from the workshop and the carburator specialist. There were several problems. The float level was not correct , although set according to the manual, and thus too lean.
Also the closing angle, also according to the manual, had to be readjusted. Then a little fine tuning and now he runs great

That is great news! Thank you for letting us know the results.

It is also good that you were able to locate a workshop who has the expertise to do correct work on these old cars.