Start with new boots. You are in plastic that is almost 35 years old and is PRONE to cracking AND/or exploding now. The plastics from the late 70's / early 80's were not good. These breakages tend to happen at the worst possible times, usually on the hill either walking or clicking into the binding. Like a lightbulb going bad, there is usually no warning.
I stopped skiing the next generation of those boots (SX91s) in the mid-90s, when they were a decade old.