Unfortunatly those bolts rust in there HARD!!! The simplest way is to cut the head off with a torch, then spread the mount enough to squeeze them out, then change the bushings as per normal. Failing that, if you can get a vise or other large squeezing device up there, you may be able to push the bolt into a socket, which will hold the brackets from bending too badly. Experience says this will be a fight. The other thing which sometimes works is to actually tighten the nut before attempting to loosen it. If the steel liner of the bushing is held tight, forcing the bolt to turn as you tighten it the rust may break loose. On a badly rusted one, the torch is likely the final solution.