Don't use sandpaper on the VIN tag. Use a Q-Tip with paint remover on it to dissolve the paint. The tag is stainless steel so you can't hurt it with the remover.
The other location is on top of the frame rail, on the passenger side, right behind the rear axle hump in the frame. Hard to see without lifting the body, but you can do it with an inspection mirror and flashlight.
1965 Impala SS Tahitian Turquoise 427 4 Speed