We have decided to take the body off the frame but are stuck on the rear body mounts. We have got the #2 mounts off on both sides but the #3 and #4 are not budging. The car is from pennsylvania and there is heavy undercoating and signs of rust. Going to try the #1 mounts now.

spray frequently with something like WD40 to make the bolts move again.
one of body mounts didn't wanna go out either but i kept spraying for weeks and having it soak.. body mount finally got free :)

Being a pennsylvania product myself, and just recently taking the body off of my 77 L-82, I found the WD-40 tip is priceless. Try soaking the bolts for at least a week, and then get in there. My #4 bolts were moshed, so I used my ever loving Sawzall on those bolts. The reinforcements were trashed anyway, and there was enough movement in the bolts to trash the fiberglass that holds the nut in with the metal keeper, so neatness wasn't an issue here.

The bolts came out in two, I removed what was left of the reinforcements, cleaned everything up, and started to rebuild. Not a hard process. I laid some extra fiberglass in the #4 opening, marked and cut, installed the reinforcements, (AFTER SANDING AND COATING WITH POR-15 CHASSIS BLACK, I DON'T WANT TO DO THIS AGAIN) and installed the reinforcements with stainless bolts. There's too much stress in that area for those tiny aluminum rivets, besides, they were all worked out upon disassembly.

The new #4 support area is my pride and joy of my restoration so far. Good luck, and take your time. I highly suggest coating the reinforcement with something like POR-15, or powdercoating before installing. That's a horrible area for kick-ups from the rear tires.

Truly an engineering feat that Chevrolet must be proud of. Almost as stupid an idea as the idiot that designed the oil filter placement of the late-80's Cadillac HT-4100 transverse-mounted motor. I still have scars on my arm from those. I would really love to meet the non-thinking person that designed this.

I've found that simply spraying a rusted bolt with WD-40 doesn't always do the trick. Something I've seen implied but never stated explicitly is to find a way(I used a plastic baggie) to keep a large amount of WD-40 on the area so that it really soaks.

Scot

It's been mentioned on here many times before, even though good ole WD is a great product, PB Blaster is much better for soaking rusty bolts, it seems to penetrate much better. Give it a try.
My #4's were hopelessly rusted too and had to be cut out. GM couldn't have made it much harder with that detail. Unless they just poured salt water in the pocket at assembly!

this is the best stuff you can buy and it's from GM-------RUST PENETRANT and INHIBITOR,Heat Valve Lubricant--#1052627 in a 10.8 can.remember those rusted frozen heat risers? this stuff will free them up--just spray on and let it soak a couple of minutes--it works better than wd40, pb blaster or liquid wrench marty

I forgot about the GM Lube. I haven't seen it for a while. Just my local dealers? It is by far the best stuff I have ever used.