If you're contemplating this project, I would loosen all the bolt snow and spray some penetrating lube down the holes right now. Then when you start the project in a week or more, at least the stuff has had some time to work it's magic. If it can...
When you loosen the bolts, leave them in, or better yet get a couple of longer ones to use as punch pins. With the bolt head up where you can get to them, hit them EXTREEEEMLY HARD with a big hammer.
If the lower washer dislodges and the bolt goes down, great! If not, keep hitting. Then hit it some more.
If no amount of hitting will dislodge the lower washer (visible from underneath the mounts) then you will proceed to Plan-B, then Plan-C, then sometimes even Plan-9 From Outer Space.
These usually include a reciprocating saw and several blades, or a decent drill motor and some large bits. You're either going to cut the mount bushings and sleeves in half (being careful not to damage the actual frame mount points of course), or drill up from underneath once the bolt is removed to nibble away at the rusted interface.
I've seen some come right out (friend removed all 8 of his in just a couple of minutes) and I've seen some take up a whole weekend. Just depends on how cooperative your mounts are.
If you have the equipment you can lift the entire body off the frame once you remove the bolts. Then wail on the mounts from a position of authority. If you're like most of us however, once the bolts on one side are removed you can use a floor jack and a 4x4 under the rocker to life one side at a time. All depends.
Whatever you do, make sure the doors are fully latched when lifting, or if no doors then the openings are reinforced to avoid any kinking of the sheet metal of the rocker.
And make sure that you absolutely positively unbolt the fan shroud from the radiator and any auto trans linkage, and maybe even the clutch linkage. Lifting the body flexes all attached points. Those are just the most sensitive to a little extra lifting.
Paul