If you can get it in the States - Plus Gas. Magic stuff. I dissasembled an entire 1977 Jaguar XJS using this stuff, and only lost a couple of bolts due to shearing. Didn't round a single nut. PlusGas is seriously amazing stuff.
A friend of mine was trying to dissasemble some very expensive aluminium suspension (ali tube, ali nut), which had been on a rally car for around 6 years - nothing would shift it; hammering, heat, WD40, none of them would touch it.
2 squirts of Plus Gas, they unscrewed right off without any effort.
I don't have shares in the Plus Gas company (whoever it is makes it). Just a delighted customer, amazed that something that comes out of a squirt can in 2014 actually does what it says on the tin, rather than being useless junk.