What spark plugs for 1963 230 engine

[dmjlambert]

Hi folks, I am gathering parts so I can clean up the engine, carburetor, distributor and coil, etc. and then I would like to see if this engine will run. The truck has been sitting in a yard since the early 90's and the spark plugs were removed. I haven't done anything serious yet except some external cleaning of mud dobber nests and cobwebs. One spark plug I found was laying in the engine compartment, it is an AC R44LTS6 and I don't know if it came out of this engine. How do I figure out what spark plugs to get? The hole where the spark plug goes has a narrow opening that would accommodate a spark plug with 5/8" hex nut, but I see some listings on O'Reilly Auto Parts and Amazon where the "confirmed fits 1963 C10" show the larger spark plug, I assume that is 13/16 nut. I cleaned the threads on the old unknown spark plug with a wire brush and put some oil on the threads of the engine and installed the spark plug into each of the holes in turn to clean up the threads on the engine.