I have one of these Westinghouse radios at my parents house but it's been years since I've seen it but it seems like it has a cheater cord socket on it which is non-polarized...seems that you would have to remove that socket and either install a polarized cheater cord and socket or just wire the cord right into the chassis.
If I remember correctly the narrow terminal of the plug is the "Hot" and the wide is the neutral, so the wire connecting to the narrow prong of the plug should go to the rectifier/filaments, wide prong to the circuit ground. Not sure that I follow the diagram you gave.
Here is a web page talking about the polarized plug:
http://antiqueradio.org/safety.htm