Firstly, nice score. I love that era of GT.
Secondly, save the flame for BBQing meat. There's plenty of options for getting the seatpost out without resorting to the torch.
There are several products out there that are better than WD40. I can't give you any brand names because I'm Australian and what I use might niot be available there, head to a bolt shop (rather than the big box hardware shop) and check out their range.
Good standbys are diesel, or 2 stroke. Leave it to soak, don't be afraid to wait. As above, clamp the head of the seatpost in a vise and use the leverage of twisting the frame to break the bond. Or hit the top of the post with a hammer, careful not to slip and hit the frame and careful not to bend the post or the extension of the seat tube. I know you want to pull the post out, but you might be able to shock it into breaking the bond, allowing you to more easily pull it out.
I assume it's an aluminium post, which means it's chemically bonded to the steel. Ammonia will corrode the aluminium, but I'm not certain what it will do to the paint.
Freezing the post with liquid nitrogen can wotk, and no, I don't know where to get that either.
A sure fire method is to get the post drilled out by a machine shop.
When it is out, get a small wire wheel on a drill and clean the inside of the seat tube out.
Good luck!
Grumps