Persistance. PB Blaster penitrant, and maybe a torch... May have to torch the heads off the bolts, and remove the whole spring pack, drive the bolt out, and replace with new, depends on how rusted it is inside the spring bushing... might be time for new bushings too.
Dunno.... hard to say without seeing it... hope this is some help, even though I might have just made it sound like a bunch more work... sometimes a seemingly easy job turns out to be much more than planed.
Good luck...