I started to remove the body mount bolts, got the fist one off without breaking a sweat. Then all down hill from there , the second one (behind rear wheel) just keeps turning.
Here is a picture showing the rear most body mount. It is a captured nut and is semi-floating.
Here is the same body mount as viewed from the rear....bumper is removed. If this mount is spinning free, use a die grinder to cut a "window" as shown by the black line. The opening will allow you to grab the free-spinning nut with locking pliers so you can remove the body bolt from this location. Next, fabricate a new nut cage and weld in. Move the "window" metal back to the original position and weld the seam closed. Use a 4" grinder to smooth the weld.
Here is the 3 body mount from the rear....there are 5 per side for Skylark models. This mount is approximately above the middle of the door sill. It is a small raised area.
Here is the raised area shown with a black circle. If this bolt appears difficult to remove, do not force it. Drill a small hole, approx 1/8" diameter and spray some penetrating oil on the bolt threads. Attempt to remove the bolt after the oil has had some time to work.
This is the second body mount from the back. It is just behind the rear wheel well in the trunk. This nut was spinning free due to the capture cage breaking loose when the bolt was being unscrewed. I had to cut open the trunk floor to access the nut and hold it in place while my assistant removed the bolt from below. The opening is sloppy since the trunk pan on this car is going to be replaced.
Here is the driver's side second from rear body mount. Notice the small hole at the top of the dome....used to spray penetrating oil on the bolt threads.
New body mount nuts w/cages are available as reproductions from www.ss396.com
If the car seems pretty rusty overall, I'd be inclined to open up all access points to any caged nuts before even attempting any unbolting. I think more bolts get broken from the initial rush to get er done. I've found that body bolts will initially begin to turn, only to seize up. Be patient here.
If you can see the caged nut from above and the bolt tip looks at all rusty, use a die grinder with a small 90 degree cut off wheel to zip off the tip of the bolt that passes up through the nut. If you cut this portion of the bolt off flush with the nut, you'll be able to back the bolt out while holding the caged nut. If you don't remove the rusty portion, the bolt will absolutely gall and sieze in the nut on the way back through, and you'll end up having to cut the bolt and will need a new nut.
If the bolt tip looks good, clean it and spray it with good penatrating oil. Then while holding the nut, tighten the bolt. Then loosen until it starts to drag, then tighten again. Work it back and forth gradually this way, and it'll eventually make its way out without breaking.
Good luck! Mark
__________________
65 Laurentian post, 67 Grande Parisienne 4 door HT.
, the second one (behind rear wheel) just keeps turning. 
