What I used back in the '70's on my 273 Bracket Racer was a steel shim head gasket (most likely specifically for the 273 bore size) along with a copper wire I epoxied into a grove in the gasket that surrounded the cylinder bore. You may consider this a poor man's version of machining an o-ring groove into the block or cylinder head. It was overlapped very slightly where the two ends of the copper wire met one another. When I disassembled the motor, it was obvious the copper wire was flat and had done its job in helping to seal the head gasket (also used copper spray on the gasket). This was not my original idea, must have read this technique in a magazine or some other written publication. Was decades ago, so I don't have any photos of this to share.
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