is that a body mount bushing?

The missing rubber fills in the gap between the frame and the body mount washer. That's why there is a space that is normal when they wear and fall apart. I believe when I looked up body mount sets the ones for the 74-76 caprice and impala convertible had the largest amount of mounts in the set and should be enough. the door pillar is the smaller one of the whole set that and the rad support may be different.

 

Drill an 1/8" hole above each body mount and flood it with penetrating oil. Do this a few times and you should be alright. A Johnston bar will give a better feel for when to reverse the bolt so as not to break the caged nut out. An impact will be faster but no feel for what's going on.
I would replace all of them rather just one as everything settled together.

 

Yea def do all of them you'll more than likely to have to pull more than 1 body mount bolt to change just 1 for clearance anyhow. Unless it's a corner mount and you can pry it down for clearance.

 

Bad news, i would say...today i checked every mount i could find. As you said...3 cage nuts were already broken by the previous owner. Great.

 

Do they spin all the way around? you do know these are diamond shaped nuts and will move a good bit before locking to remove/tighten. There are no caged nuts on the rad support, they are nuts and bolts. You can access the caged nuts from above if they are spinning but you will have to do some chiseling. the 2 inner front BODY mount bolts can be accessed from under the car or you can cut the floor. If I remember correctly they do have access holes but are a real biotch to get to. If you CAN get to em (inner fender removal) you can simply use the Kentucky fire wrench and cut em then use nuts and washers on the top.

 

If bolts brake off and do NOT spin in the caged nut remove all the mounts and lift the car and heat the caged nut red hot around the bolt use vise grips it should come out. I know it sux man but that's how its done

 

If one plans to live to be 300 years old or keep the vehicle within the inheriting family, it might be an idea to replace those bolts with studs, being that rubber chemically reacts with environmental factors, depending on exposure. Keeping the vehicle in a cool, dry and dark location will preserve all rubber components.

 

Three nuts are definitely broken, I can feel it.
I'll have to think about all that and have a talk with my cousin who is able to weld.
I cannot take a pic of the radiator frame mounts, but these are not normal bolts and nuts, I'm pretty sure that's also a caged nut. I'll try to remove and replace them with a normal nut and a big washer... should be good as well.

 

My condolences to the family jewels.

 

Don't overlook a possibly simple cause - loose jack equipment knocking around back there.........