Replacing the head gasket is not a difficult job but I wouldn't advice doing it without a proper manual to guide you. You can get the MB manual on CD form the FastLane link at the top of the screen or directly from MB.
I would NOT give the head bolts another 1/8th turn and risk breaking a head bolt.
Please check the archives on the OM603.971 engine before planning that long trip. Statistically, there are few head and head gasket problems with this engine and a lot more bottom end problems. Similar symptoms can be caused by a bent connecting rod. I hope it's not that and that you can get by with replacing the head gasket.
I'm sure quite a few responses will follow as they folks here like to comment on the 350SD/SDL.
BTW, how many miles or kms on your engine?