I've got one out here out of a 55/59 truck that's easy to get to but it's too late an too cold tonight.
I think if you look at the threads, they are fine threads and the plug has a sholder on it and takes a gasket.
Ecklers shows this one for a car
http://www.classicchevy.com/chevy-re...1957-1964.html and I think the truck uses the same one but can't guarantee it.