I've just discovered the rearmost chassis/body mount reinforcement on my 80 has rotted out. I'm talking about the pressed steel square shaped cup that rivets to the fibreglass, then has the body bolt through it. I know these are available, but I'm just scratching my head about the easiet way of putting a new one in with the body still on the chassis. By my estimation, the body might need to be lifted a couple of inches over the mounting point, to allow the new cup to be slid in.
Has anyone had to do this, when not in the course of a body-off?
If I was to release all four of the rear mounts (the two rearmost and the two behind the access panel in the wheel well) would this give me enough "give" in the body to lift the back up and slot in the new reinforcement?

 

The tough part is gonna be getting the bolt out, the captured nut usually is rusted and the capture part doesn't hold. LOTSA Liquid Wrench.
After the bolt is out you can try using a block of wood and a jack to seperate the body from the frame. It will move a little, cut the old one out. You may have to bend the new one to get it in then reshape it when it's in place. Then rivet it or use sheet metal screws. When I did mine I used some silicone adhesive/sealer with sheet metal screws.
Then undercoating.
Good luck.

 

jesse is right about getting the bolt out, they are rust stubborn. The number 3 body mount bolts inside the inspection plate are usually difficult as well. The little cage inside #4 mount only keeps the rectangular nut from turning. I made a rectangular washer from 3/16ths plate to match the indent created by the original and tightened it down with a hex nut and lock washer. Also, I fabbed a sheet metal inside support similar to the original from a cardboard template. A hole saw was used to cut an access hole in the center for the body bolt. The mount holes were drilled out to 1/4 inch and I used s/s phillips, pan head screws with nylock nuts to hold it all together through the body. This replaced the original rivets which are difficult to replace and not as durable or strong. Working from the inside on the #4 mounts is a real PITA. Use a mirror to look down into the recess, it will improve seeing the work area. Removing the #3 and 4 body bolts should give you enough slack to replace the #4 outer mount. Before you try and raise the rear, you will also need to deal with fuel lines, bumper brackets, wire grounds, shift linkage, etc. Don't overlook these things or you could make more work for yourself later. Go slowly and think everything through. You can probably see why many body off resto's started just like your smaller project.

 

For an 80 - there is already a bit of room due to the thickness
of the rubber body mount. The fiberglass has enough flex that
you should be able to squeak it into place without removing any
of the other mount bolts.

I would go at the old bolt head with a 4" grinder, if the cage nut
just spins. Unless you are building an NCRS candidate, you don't
even want to use that cage nut. Just use a bolt - down from
the top and a nut on the bottom.



I just did one (body off). Check out my post:
http://forums.corvetteforum.com/showthread.php?t=872065

 

Hey guys, cheers for the info so far.
Perhaps the best technique for me then, is to loosen both #4 mounts (although only 1 side is "terminal" the other doesn't look too clever!) and just see if I can make enough room with only those unhooked. If yes, all good and I dont have to tackle the #3's. If no, then I'll have a beer and cigarette break and battle on.
As far as disconnecting things, the entire back end is stripped out now, including fuel tank (found my build sheet - hooray!) so I think we'll be ok for interference issues. I'll have a good check further up to the front before I do any jacking though.
This'll be fun