Don't spread the frame, that's probably not where the problem is.
Start by using a penetrating oil like Kroil, or similar. Or make your own with a 1:10 oil:mineral spirits mix. Paint a mark on the post as a reference to see where you're going.
Now twist the post by the saddle while pulling up. Or remove the saddle and put the post in a vise and twist the frame while pulling it back.
The key is to keep it oiled, and work slowly to prevent heating. Check the frame for heat, and if it's hot to the touch, rest it until it cools. With patience, you'll have the post out with a minimum of untoward consequences.
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