About the only way you can get the headers bolted on is to drill and tap the two end holes that are not drilled in the heads. (This is not all that easy to do with the heads still in the car.) If the heads are in the car and you remove them, there is almost no reason to keep them. The #11 heads are 1970 350 heads. These are for low compression motors. They have small valves and press-in studs. With the kind of gas you have available in Germany, I would recommend looking for a different set of heads that will give you more performance and have those holes drilled. What size motor do you have?
If you wish to use those heads, you must drill and tap for header bolts.
Use the headers as a template. Bolt them to the heads with the other 4 bolts. Center-punch the two outer hole locations, then drill and tap the holes. Drill with an 8mm drill and tap with a 3/8-16 UNC-2B tap. Make sure you drill as straight as you can.
