On that old of a vehicle I highly suspect that the charging system the battery terminals, starter , alternator , fuse box power junctions, engine block ground and grounds are corroded or rusty.I'd disconnect them one by one and clean them with a wire brush , sandpaper , a file and even clean the o-ring wire connectors , be sure that the chassis etc ground surfaces are well cleaned too and also either clean or replace the mounting bolts and screws etc.
You might want to start with the main thick wires that run from the battery to the starter , alternator , fuse box and engine block ground bec their probably corroded you'll notice a green or gray powder that is a clear sign of corrosion and more then likely these main wires are internally rotted or corroded.Their not too expensive and if you can universal replacement wires from many different auto parts stores from 10 to 20 bucks or so a piece , just remember to clean the surfaces and connectors that these wires connect too when you replace them.
Hope that helps and best of luck.By the way my gut says that doing what I've mentioned will fix this problem but remember that even a fuse or fuse socket builds up a dust that causes bad connections so you might want to clean them and any relays etc.It not hurt to perform a basic tune up of plugs , air filter , gas filter etc.
