If the bolt is siezed then it'll be siezed to the inside of the bush and you'll have to hacksaw through the bolt between the radius arm and the axle bracket, otherwise there won't be enough clearance to get the radius arm out.
It's also best to have a new set of nuts & bolts to put in when replacing the bushes just in case this happends or the old bolts are worn & loose in the new bushes.
Give the nut on the chassis end of the trailing & radius arms a good soak with WD40 before you try to undo them, I snapped the end of a stubborn radius arm once! :(
If you undo all the nuts & bolts on one radius arm then lever the axle end down and clear, the chassis end should just pull out. Do the bushes, re-fit and do the other side. Same goes for the trailing arms on the rear axle.