1. They dampen/eliminate vibrations in the body.
2. Yes. 4 in each car. Front ones are between the radiator support and inner fender, tucked inside the fender. i.e. behind the battery. (at least on my 69)
3. I think 67-8 are the same but not sure on that one. 69's only fit 69.
4. They weigh about 25 pounds each.
5. Big NO here. You'll have to find a set of used ones. They are around, but can be hard to find. A set of 4 for a '69 go for about $400.

Hope this helps, Good luck.