NAPA online shows 3/8" for all plugs listed for your year. Did you get NAPA info from online or from a local store?

I have several excellent NAPA dealers who have people who can and will look into catalogs for old parts. Nonetheless, I check out parts in NAPA online before I call or visit a local NAPA store.

Maybe someone will post informaton about the longer plug being OK. I'd go with 3/8".