Ah, that old chestnut. It's exactly what happened to me and it is quite common. It may still loosen under all that plusgas, but if not then I can only suggest the following.
On the assumption that you are not a professional and are doing this in your garage or on the drive; first - don`t over force it until you know what the exact problem is. It can snap (that is what I did).
Second - don`t use heat. The only way to get that much heat into the bolt to free it up will melt the bush of the transverse arms - if you don`t set fire to them first.
You need to check exactly where it is seized. For me it was the anterior transverse arm bush and the metal insert on the bush had welded to the bolt. As an experiment when I finally got it off I tried a cold chisel and a lump hammer and still couldn't shift it so they do seize on tight. Try to turn the bolt and fiddle with everything it goes through to see exactly which bit it is seized on. If its a bush then you should see the rubber twist slightly.
If the transverse arms are free and it is seized in the centre spacer between the shock strut and the hub then IMHO I would deliberately snap or saw the bolt, free off the transverse arms and take the whole strut assembly off. Get a new bolt and drift the old one out on a bench or use a hacksaw to saw the remainder off. You might be able to saw the bolt off entirely in situ but it will probably take a very long time as space is tight.
If its the posterior transverse arm bush that has frozen, get under the car and remove the nut at the other end of the arm at the crossmember to take the arm off with the bolt. You will probably need a new arm but they aren't that pricey. (Actually I have a brand new spare one in my garage if you need it
If its the anterior arm then be warned, you have to take the entire rear suspension crossmember off the car to remove this arm. That or drop the fuel tank and you don`t want to do that really. Its because the arm will not come off the bolt unless you pull the bolt out towards the back of the car which is unlikley as it will not want to move and you have no purchase on it. It's actually quite easy to drop the crossmember but it does mean you are committed to fixing it yourself as you are effectively removing all the moving parts from the rear of your car and it's not going to be transportable to a garage, even on a flatbed, if anything goes wrong.
If you need a new long bolt then I still have a spare from when I did mine, plus a few of the washers and the 'spacer' between the strut arms and hub. Also a few crossmember bolts. You might be grateful for them as Alfa will charge you about £7 for the bolt and a few quid for the washers too. Daylight robbery.
Good luck and i hope its not as bad as to need any of this and that in the morning it will decide to come off after all
PS. you are using a breaker bar aren't you? You wont get far with a standard socket set.