You have the right idea with the heat but probably have not used enough heat.
Remove all of the pot metal pieces (airhorn, and bowl cover)
Remove the float.
Forget the penetrating oil until:
Take an acetylene torch, and heat the cast iron area around the plug until it glows red!
Turn off the torch, put down the casting, and go do something else for about 15 minutes DO NOT QUENCH THE CASTING IN WATER
Now after the 15 minutes have passed, and the casting is cool enough to handle in your hands without glove, add a little penetrating oil and remove the plug.
Jon.
