Heat is good. Boiling water is quite effective and idiot proof, if you don't have a suitable torch.
Freeze spray from your local Jaycar (etc) on the bolt can also help, with a thermal shock to help break the bond.
Once you have the nuts locked on (tee hee), tap the end of the stud with a hammer - short, sharp cracks will help loosen the stuck thread.
Worst case isn't that bad - cut it short, then drill the bolt out. If you're lucky, the bolt will screw into the swingarm without damaging the threads in the swingarm. More likely, you have to run an 8mm drill through it and then helicoil it.
Put grease/anti-seize on the new bolts!
Early 90s YZ?