Thats too much for the car in my opinion, but reasonable assuming the body shop is doing it right.
I bet I have $3K in paint materials on a Camaro I am doing. $8,000 is nothing to fix a rusted out car.
The Chevelle you are looking at has been worked over at least once. So for every flaw you see, there is still 10 more underneath waiting to surface.
I like your choice in cars, just get it bought for 1/2 or look elsewhere.