It could be 76, though I'm not sure if they still had the impala out by then (They were definatly still using that base style in 76) What year does the engine have stamped, and does it seem like it is the original engine? (I dont think impala's had anything but white engines)
Grumpy, as long as you are carefull, these doors will last, depending on the area, and how they are kept, they can rust like this, but occasionally brushing the grass of the back of them, and keeping the mower reasonably dry will stop it happening.
