I wonder if the 50s ignition switch would have gotten hot if the radio had been connected to the ammeter post and the heater motor lead had been connected to the "dead terminal" on the ignition switch.
I would bet that most installers did not follow the installations instructions, and they simply wired every accessory to the ignition switch.
As stated above, a fuse block and/or relay(s) are good, but the ignition (key) switch is not the only place where electrical power was (supposed to be) obtained in an original configuration.
