In the older vehicles, only the radio (and other installed electronics such as a radar detector) along with the diodes in the alternator would be affected. Why a magneto/generator ignition system would survive more readily than an alternator type system.
Would a well grounded .30 or .50 cal ammo can be enough faraday cage to protect a couple of rebuild kits with diodes for the alternators? Or would the protection need to be more elaborate?