[Arran]

In my opinion there is very little electrical hazard from working on a floating chassis AC/DC radio, unless you intend to power the set up, and touch the chassis whilst standing, bare foot, on a concrete floor. Adding a polarized plug is a waste of time, once the chassis is inside the cabinet, which is either made of wood or Bakelite, nobody will be making contact with the chassis if the set has a back on it. I worked on dozens of AC/DC sets over the years, my habit was to only plug the chassis in to test it, then leave it unplugged the rest of the time. Even whilst plugged in you could only feel a current with the chassis powered on, plugged in one way, but plug it in the opposite way and you could feel a current when the chassis was turned off. You could invest in an isolation transformer, or just exercise common sense.
  I don't think the IF cans are the problem, I think it's in the local oscilator. Is the loop antenna connected whilst you were testing the set?
Regards
Arran