Along with the heat, give it a couple good whacks to break the corrosion free. Anti-seize works good when putting them back in, just keep an eye on it so it doesn't come loose, i'd rather put a wrench on it and snug them up while the sled is warming up each ride than mess with that again.
I broke one off once trying to get it out, ended up welding a nut on the end of the busted off part and that took it out.
If you can somehow get to the inside of the tube that the bolts screws into you could squirt some "Sea foam" into there and let it sit for a while, that's some of the best stuff I've used.