I have a 70Z that did the same thing as you describe. It was an ignition wire. Don't recall exactly which one, but they were stock wires and just broke. I would check out the igniton system first.
I have a 1970 Z with the original 4-speed. I did purchase a 1982 five speed but after considering the changes that installation would require decided to stick with the 4 speed. I have heard the original 4 speed was a stronger transmission but really don't know.