If the pan and plug are original to the OEM engine, the thread size should be 7/8" and the correct washer (crush) should be copper. The drain plug threads (and possibly pan threads nay be stretched.

Try a new washer first and if they doesn't stop it find a new drain plug and maybe chase the pan threads,

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Replace the washer on the drain plug 

You might try Permatex Aviation Form-a-Gasket on the threads. 

My 56 Victoria with a 292 has the large size drain plug on the oil pan, I think 1 1/16 or 1 1/8. I have been putting up with the quarter size marks on my driveway and have wondered if modern oils now go where the old oils didn't. I have tried replacing the copper washer with a neopreme one and when that still leaked,  I tried a red silicon sealer smeared on the neopreme washer. It still leaks . Can anyone tell me if they have found a good fix for this?