The reason I am on the internet looking up Zinsco panels is because last Friday our company went on yet another service call for these dangerous panels. Not only did the branch breakers in this panel not trip neither did the main. Infact the intire city block was knocked out before fire quit shooting from this panel. This is the 4th house fire I have been witness too as a result of Zinsco panels. Our local inspector will not allow any alterations to a home with a Zinsco panel and carries a burnt up panel in his trunk to show people.
We as a contractor have changed out over 50 of these panels and I will not sell any parts to anyone. They are just too dangerous. In answer to your question is there a safe way to add gfci protection to a Zinsco panel the answer is no.