A quick and easy thing to check is the power junction block on the headlight side of the radiator support by the Battery. This is the main power feed for everything but the starter motor and tends to melt corrode in a 50+ year old car. Look it over carefully and the wire that feeds it from the battery as it can show 12 volts across it with a volt meter but under load can lose voltage/amperage if in poor condition. If ok follow wire across to horn relay on engine side of radiator support on drivers side near the top next to the radiator. Here is the factory manual, go to section 12 chassis electrical and once there scroll down to the Firebird diagram as it's mixed in with full-size Pontiacs and Tempest. https://www.firebird400ho.com/mdocuments-library/ If it wasn't for your description of the lights and radio dying I would have said coil but by your description you are defiantly loosing an electrical connection. And of course as always check your grounds.