I doubt it's the head gasket. The m116 and m117 engines almost never experience head or head gasket failures if they weren't overheated. In order, I'd check/perform:
1) TIMING
2) Sender gauge accuracy
3) Coolant level and air in the lines
4) Coolant flow (water pump bad?)
5) THOROUGH block and rad flush and replacement with water, then drain again and replace with coolant.

If your timing is off your engine can run pretty damn hot.
Edit: To clarify, that's ignition AND cam timing. The m116 and m117 engines have a "weak point" in their plastic timing guide rails. If they snap, your cam timing will be off. If you're lucky, you'll experience poor running or problems like high temps. If you're not, you'll have engine failure.

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Back in the saddle - 1972 280SE 4.5