Just make sure you check the port configuration before use.
I think there is one manufacture stamping those gaskets for several after market suppliers.
I see your point as the 1968 340 Exh. Gasket port size I think is 1.25x1.75
found this Exhaust Gasket Size..................1.14" x 1.59" for 318/273
I wonder if they can be trimmed ?
I see these but not sure how good they will last. Anyone use these> They list the correct port size. Remflex Inc. 6027 Remflex Exhaust Header Gasket Sets | Summit Racing