How to articles; classic car restoration
How to know when a classic car is restorable.
Page 6
Body mounts
Body mounts on a 64 Impala aren't typically a problem unless the car was driven in the country for most of it's life. Even country driving is not the worst, but if the classic car was driven on dusty or muddy roads, the body mounts can become caked with dirt. The nooks and crannies, just like on english muffins, can be storehouses of mud and moisture, which always leads to rust.
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There are 3 body mount points in the rear of the car that can really make a quality restoration a challenge if the body or chassis has rusted around the body mount points and weakened them to the point where they collapse under the weight of the vehicle. The problems that this kind of damage can present might not be immediately known when looking at a classic car for restoration. The alignment of the body panels such as the spacing between the trunk deck lid and the quarter panels, or the horizontal misalignment of the rear bumper with the bottom edge of the trunk deck lid can be signs that the integral support of the chassis is compromised. A leaky rear window can be another sign of body mount rust. The chassis can sink down to the frame in the rear and stretch the rear window opening in the process.
There is a body mount on the outside of the frame rails between the rear wheels. This body mount supports the chassis between the wheel wells at the upper trunk deck. 
It's fastened to a box frame that is integrated into the chassis, with a rubber cushion (bushing) in between. The rubber will dry and harden over time and when the box frame rusts out, the box frame will collapse under the weight of the chassis. 
The box frame is susceptible to rusting out since it has some "vent" holes just behind the rubber body mount bushing that you can see in the pictures. I've taken a picture of this body mount from the front side between the rear coil spring and the rear tire, again from just behind the body mount near the tail pipe hanger, and from the rear inside of the rear bumper.
The other body mount that can be a problem is at the end of the frame rail where the rear bumper is mounted. This body mount is also fastened to a box frame integrated into the chassis. 
The box frame runs the width of the car parallel to the bumper just below the trunk deck. This body mount is prone to rusting for the same reason the rear bumper filler panel will rust out, and will also rust out when the space it occupies becomes caked with dirt. This is a difficult body mount to visualize so I have taken some pictures from a few different angles. 
First a picture from directly under the car shows part of the rear bumper and the end of the frame rail with a semi-circular cutout with the diagonal bumper bracing. The second picture is taken from under the rear quarter panel. I also shows the frame rail and the diagonal bumper bracing. This is the angle that the third picture is taken from. The third picture is a viewpoint you could not get without a small camera. Holding the camera up to the underside of the trunk deck allows you to see the rubber body mount bushing between the box frame under the trunk deck and the frame. This is a viewpoint that is perpendicular to the diagonal bumper bracing in the first 2 pictures which I have noted in the second picture.. Next page (Trim and Molding)