The troubleshooting guide or the DVA guide from this website: cdielectronics.com is an expert site for cranking voltage outputs for your 1977 engine. These cranking voltage outputs are a great measuring tool. The factory did not start using them til around the early to mid 80's. You said you have been testing your stator and timer base. This website indicates that the cranking voltage outputs from the stator must be between 150 and 400 volts. As measured by a special "peak reading" voltmeter. Anything less than 150 volts cranking and the stator is condemned. The timer base must output 0.6 volts or it is condemned. Once you qualify both the stator and the timer base, the next most likely suspect is the power pack. The spark plug coils are generally reliable, and it's unlikely the other 3 coils have all failed at once.