[Arran]

 I'm not sure about this model of Hallicrafters but on the S38s I own they went to extra lengths to isolate the chassis from the cabinet, and they had a floating common negative in the sets top begin with. That gear reduction looks like an idler wheel from a phonograph, I'm sure that one of the previous owners added that, if Hallicrafters would have added a gear reduction I'm sure they would have installed it on the inside.
 I'm with Codefox on the isolation transformer, though I would use an outboard one, (a GFI outlet would be another alternative) I'm very skeptical about playing around with the polarity on line cords making an AC/DC set any safer. It's been my experience that there is often a current flowing through the chassis with the set shut off even though there is not when it is on, and that's with sets with the isolation cap between the B- and the chassis. 
Regards
Arran