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Yesterday I was unbolting everything off my 77 to get ready for the body to come off. I got to the back bodymount bolts and they are completely rusted up. The bolts will not come out. I know that I'm going to have to cut them out, but how do you fix it when new mounting hardware is installed?
I've been through the same thing on both of my vettes. :) Fun isn't it? Cut the head off of the bolt. Just the head, then when you take the body off the bolt will still be sticking out an inch or so. The metal mount itself on the frame will be rusted out. Once the body is off you will have to cut the top out of it and weld in a new piece with the proper sized hole in it. Soak the broken bolts down with Sili-Kroil (penetrating oil for those of you that can't get the best) and leave it for awhile. You will have to get inside the back of the body behind the speakers to soak it down. After awhile the bolts will come out.
I replaced them on my nearly identical '72 by grinding out the aluminum rivits first, then attacking the bolts after the steel side plates were out of the way, used a sawzall on it, put new shims in place....
I think newones are available now, try Zip Products....or good used set as I did some years ago....when I did mine, I bolted them in with backing/fender washers, and bolts with lock nuts....while holding the body up on a 2x4x8 with floor jack...be careful there as you don't want to crack glass....I remeoved all bumper bolts, but not sure on your '77, you may be able to get away with not touching the rear area at all,....after the new ones are in place, I poured in some 'glass resin to seal hell out of them internally, and 'buck' that steel plate against the glass....them spray hell out of it with aerosol undercoating, several coats....tons of fun, messy job..... :U