You drove the car a fair distance before the seize, and surely you would have known if the engine was running on 5 cylinders. I tend to agree the spark plugs are a real mystery, but can't believe you wouldn't have detected that severe of a misfire if the gaps were closed from the first fire-up. Especially with a manual transmission and clutch where everything is transmitted to the driveline.
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Hoping to finish a project while I'm still able to push the clutch in.... 
1963 Tempest Convertible (Ram Air IV w/4” stroke), TKX, 9-inch w/3.70 gears and triangulated 4-link) Pictures
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