You can probably lift just the corner then, but personally I would still loosen all the bolts so you're not trying to twist sheet metal even that little bit.
Loosen all 8, but the side you're working on at the time get loosened more. Lift it the amount you need, slip the shims in and bolt it back down.
Can probably leave it at 25lbs or so, but that's just a guess. I'm sure that size bolt could go all the way to 40 or so, but that seems overkill for the use and I believe they are coarse thread anyway so should not be maxed out if possible.
Are they the stock original body mounts? If so then the mounts will stay clamped to the frame the body will come up off the top of the top plate of the "clam shell" arrangement. If that's the proper term.
With stock mounts you can undo the bolts and lift the body straight up and the mounts will all stay in position even if you try to knock them around. They're clamped but good!
Paul