I have not done this this way as I pulled the body off my frame in doing my frame up rebuild, but this is how I might do it.
You don't want to lift the body up 3-4". You will probably damage sheet metal at best. Loosen all the frame bolts so you have a little play under them. Then remove all the bolts on the side you plan on replacing the bushings. Put a long 2 x 4 under the rocker panel and raise it up just enough to sneak out the old bushings and install the new. From what I have read, the hard part may be getting to the nuts that hold the bolts on the firewall bushings as the inner fender may be in the way.
Reinstall the bolts on the replaced side, but leave them loose so they have a little play. Then do the other side. Install the bolts. The look at your body lines to make sure the body has not shifted. If it has, then adjust the body to get your body line squared up again. I would take pics of all the body lines so as to use them as reference and duplicate them if the body shifts on the frame.
So survey the car up on the lift and see if this might work for you.
The broken bolts will most likely not be an easy fix. You will have to get at them through the top, ie the floor pan & under the carpet. You will have to cut an opening/window to see them. The bolts are typically held in by a cage nut that is welded in. You will then have to cut out the metal/cage nut to remove it and the broken bolt. Then cut a piece of new metal to be welded back in with the hole for your bolt to go through. Then weld a new cage nut on to that piece, and then weld your hole up in the floor.
Although this is a '68 Chevelle, it may give you an idea of what I am talking about. Body Bushing Replacement - Chevelle Tech
Study your situation and see if the above fix will work. You may come up with a better idea, but this seems to be the typical fix/repair when these bolts either snap or the cage nut spins loose.