When you press the starter button, it grounds out the blue/white wire connected to it, so that's the connection I'd be checking, along with the ground wire running to it.
When I got my '78, the starter was totally dead. Turned out to be nothing more than a rusty connection of the heavy wire on the starter motor. The rubber protection boot had split, allowing water in, and it just got all rusty. I cleaned it up (removed the wire and wire wheeled the end of the cable and the stud on the starter), installed a good used rubber boot, and it's worked fine ever since (nearly 20 years now, lol).