Yep HEAT and plenty of it.........I always start with PB Blaster and if possible soak it and let it soak/penetrate for an hour or even a day. Then HEAT, HEAT, wait for it to go cherry red if you can. Then a quarter turn loosen and a quarter turn back, 1/2 turn loosen, 1/2 turn back and so on.....sometimes even if I have the bolt halfway out I'll spray the heck out of the exposed thread with the PB Blaster and run the bolt (or nut) all the way back in to clear the thread of any rust or corrosion, then spray it again and then bring it out to where it was before you ran it back in and repeat the turn out and turn back in, all the way until it is finally removed. HEAT and 1/4 and 1/2 turns out and back in. If you rush it you will bind, strip or break the fastener every time. Good luck

One of the bad things is the tip of the bolt that sticks out of the captured nut inside the channel usually has the worst rust and corrosion and you're pulling that part through the nut. If you have the ability to wire brush or clean or even burn some of it off with a torch that will greatly help on some jobs.