That's not the correct casting number for a '58-62 head, which does have 1/2" NPT threads. An earlier 235 head may have a 216 style thermometer opening which is like a 3/8" female gas tubing flare. It does not use pipe taper threads, they are straight cut. You may not have an easy time finding a plug for this, although you could use an adapter to go to 1/2" NPT and then plug it.
