There is a spark plug article on the ponton page:
www.mbzponton.org.
Absent technical data to the contrary, I would gap the plugs at 0.032 inch. How many cylinders does your car have? What does your manual say?
The choice of plug brand is mainly one of choice and personal experience. Some people sing the praises of NGK, others like modern Bosch. The heat range is perhaps more important. Why are you using Bosch W8 heat range? It may be the correct one, but how have you determined this? What was the orioginal Bosch plug specified in your owners manual? I have concluded, from hard experience, that there are too many errors in the technical data contained in the Glenn book to place heavy reliance on it. This is particularly true of its pages on spark plugs.
Spark plug conversion tables are notoriously inaccurate.
I sense that, generally, the consensus re older Mercedes is not to use Bosch platinum plugs, just regular plugs. The exception may be fuel-injected Adenauers, but I am not certain.